This sinkhole has been known to local fishers for centuries. According to local legends, this is the eye of the South China Sea and here the Monkey King found his golden mace.

In 2015 – 2016 a team of Chinese researchers (Sansha Ship Course Research Institute for Coral Protection) explored this unusual place, robot "Video Ray Pro 4" reached the full depth of the sinkhole.

Upper part of the sinkhole is rich with sea life but at the approximate depth of 100 m starts a layer of oxygen-free water which, most likely, is devoid of life.

Dragon Hole has formed in the massive Miocene deposits of limestone which are more than 1200 m thick (1). It is not quite clear how such a deep underwater sinkhole was created.

Currently (end of 2016) fishing and tourism in this area has been prohibited.

References

1. Shiguo Wu, Xinyuan Zhang, Zhen Yang, Tuoyu Wu, Jinwei Gao, and Dawei Wang (2016). Spatial and temporal evolution of Cenozoic carbonate platforms on the continental margins of the South China Sea: Response to opening of the ocean basin. SEG Interpretation, August 2016. doi: 10.1190/INT-2015-0162.1
2. China Exclusive: South China Sea “blue hole” declared world’s deepest. Xinhua, 23 July 2016. Accessed on 22nd December 2016.

Dragon Hole is included in the following list:

The legends of Ye’kuana Indians gave the name to Sarisariñama tepui (1, 2) – a spectacular, forested table-top mountain in one of the most remote areas in South America – south-eastern Venezuela.

It was only on November 25, 1961 (3) that a famous jungle pilot Harry Gibson happened to fly over this mysterious mountain and spotted something unusual – a carpet of beautiful green forest that was perforated by two enormous and apparently very deep holes – each having a patch of forest at their bottoms.

In the fantasies of many people this unusual natural phenomenon promised exciting discoveries – maybe prehistoric animals still are living there?

Forested tepui

Sarisariñama tepui is located in the middle of a primeval forest with the closest road being hundreds of kilometers away. There are more than 100 tepuis (very impressive table-top mountains) in the southern parts of Venezuela and Guiana as well as in the bordering regions of Brazil. Tepuis consist of pink, arenitic Paleoproterozoic sandstone (quartzite) of Roraima formation and belong to the oldest exposed formations on Earth. Their age is approximately 1.8 billion years (4).

The summit area of Sarisariñama tepui is 546.88 km² and its slope area is 482 km² (5).

Most tepuis do not have forest on their tops but in this respect Sarisariñama tepui is not typical. The jungle on the top of this tepui is so dense that one can pass within 50 meters of the rim of both sinkholes, without noticing them.

In general tepuis in the eastern part of Guiana Highlands have some sparse and low forests and are comparatively poor in species diversity, but Sarisariñama tepui is an exception, having 15 – 25 meters high trees and being rich with varied species (5).

In total Sarisariñama tepui has got four biotopes represented (5):

• Tall, upland scrub on rock
• Tubiform, shrubby highland meadows on rock and peat
• Medium, evergreen upper-montane forest
• Medium, evergreen montane forest

Sarisariñama tepui is characterized by endemic species such as the frog Stefania riae Duellman&Hoogmoed, 1984.

First explorers – Indiana Jones-style adventurers

We know too little about the history of Indians in South America and can not be sure whether they have ascended the mountain and reached the sinkholes in the past.

The modern world had to wait for 1961 and a jungle pilot named Harry Gibson to spot both sinkholes or simas as they are called in South America.

The announcement made by Harry Gibson about the weird formations in Sarisariñama tepui was well-known among specialists and adventurers. But walking hundreds of kilometers through the impenetrable jungle to reach the base of Sarisariñama (one Norwegian expedition (2) in the 1990s made only 300 meters a day through this jungle), ascending many hundreds of meters of steep cliffs, going through the jungle again, and descending to an unknown depth required strong will, organizational and managerial capabilities and… a lot of money.

Only in 1974 was the first expedition to these sinkholes organized by explorers and adventurers David Nott and Charles Brewer-Carias, involving numerous specialists who used this lifetime opportunity to be the first in an unexplored area.

The expedition used a helicopter and thus did not have to spend too much time and effort to reach the summit of Sarisariñama. Groups of people were released into the jungle next to the largest sinkhole and three men descended on 7 ropes tied together, each 50 meters long.

As they descended, they realized that getting back was not going to be easy because the sinkhole down below was wider than above and the rope was hanging free. Thus explorers were in a trap, like bugs in a jar. During the next few days, a winch was arranged and a chainsaw and other supplies were lowered. The three men made a desperate attempt to cut giant trees to make an open space for the helicopter, living with the unpleasant exhaust of the chainsaw. This slaughter of unknown and most likely, very rare species of plants was unnecessary, as, in the end, they used cable ladders to get out of their trap.

During their stay the expedition looked for the underground river which had formed the sinkhole, but was without success: there was no water body below.

More thorough investigation of both sinkholes was done two years later, in 1976 by a joint Venezuelan – Polish expedition. They discovered one more sinkhole, Sima de la Lluvia. For some two decades, it was the longest known quartzite cave (1.35 km) in the world and its exploration to a great extent solved the mystery of the formation of these sinkholes. In total there are four sinkholes known in Sarisariñama.

Sima Humboldt and Sima Martel

After some controversies, both sinkholes were named by a participant of the expedition from 1974, Eugenio de Bellard Pietri in the noble names of the explorer Alexander von Humboldt and the legendary speleologist Eduard Martel – Sima Humboldt and Sima Martel. Often the simpler names are used instead: Sima Major (Large Sinkhole) and Sima Menor (Small Sinkhole).

Sima Humboldt or Sima Major is the largest one. Its depth is 314 meters, with a volume of 21 million cubic meters. At the upper rim, the sinkhole is a maximum of 352 meters wide, but down below is up to 502 meters wide (7).

Sima Menor or Sima Martel is 248 metres deep.

Both sinkholes are roughly circular in form, but not exactly round. The distance between the rims of both sinkholes is approximately 700 m.

Sima Menor contains stalactites of exotic black mineral called litiophorite. In both sinkholes, there are small stalactites, mats, and coralloids of opal. The formation of opal has been initiated by bacteriological processes (8). In Sima Humboldt, one can find gypsum crusts as well.

The vertical walls of both sinkholes have been insurmountable to plants and animals living on the bottom of both wells. Thus, over a long time, there has developed a unique, small ecosystem with such species of plants and animals which are found nowhere else on Earth.

It takes some hundred million years and a bit of water…

Sarisariñama is not the only place having such sinkholes: there are similar formations in nearby tepuis; such as the breathtaking and deeper Sima Aonda on Auyán Tepui. Such sinkholes have formed through the following steps:

• Gradual chemical and mechanical erosion of the hard quartzite along the fissures;
• Formation of voids;
• When the area of voids increases and the ceiling becomes unstable: a subsequent collapse of the ceiling until the surface of tepui has been reached.

Both of the Sarisariñama sinkholes are located close to the rim of a tepui and there is not much doubt that here the process has been the same. This process has been very slow and a large stream was not needed to create these giant holes (9).

Such processes are unusual because the quartzite forming tepuis is impermeable and with exceptional resistance against water erosion. But tepuis belong to the oldest mountain formations in the world and here time is measured in many hundred million years. The sinkholes of Sarisariñama tell us: if there is enough time, water finds its way through the hardest rock.

The geological processes in the Sarisariñama tepui resemble karst processes in many respects, which are typical for the much softer and more soluble rocks like limestone and dolomite, but are a lot rarer and take a lot more time.

Living legend

The legend about the Evil Spirit is alive up to this day and many Indians still are afraid of the jungle-covered tepui.

The entire Sarisariñama tepui and surrounding areas are included in Jaua-Sarisariñama National Park and the tepui is off-limits to general tourists. Only researchers may get a permit to visit this unusual location: the unique nature of this site is too unique to afford the risks created by tourism.

But it is also true that visits to this unusual site are offered by some adventure tourism companies – Wondermondo is not sure who is cheating whom here.

Myths and truth

A wish to experience sensation and the “copy&paste” culture has created some myths about the sinkholes of Sarisariñama tepui. It is about time to dispel some of them:

• Sinkholes of Sarisariñama are the largest/ deepest in the world. Not true: for example, Abismo Guy Collet in Brasil is even 671 meters deep and the Sima Aonda (Venezuela) is 383 meters deep.
• Both sinkholes are perfectly round. Not true: both Sima Humboldt and Sima Martel are elongated and not even in form.
• Mechanism of the formation of these sinkholes is a total mystery to scientists. Not true: it has been pretty clear since the expedition in 1976.
• Sinkholes were discovered only in 1974. Not true: they were discovered in 1961 and first accessed in 1974.

References

1. Lindsay Elms. Mount Roraima: An Island Forgotten by Time.
2. Travel Explorations. Norwegians returned alive from the Devil’s Cave., 2003-12-18.
3. Otto Huber and John J. Wurdack. History of Botanical Exploration in Territorio Federal Amazonas, Venezuela. Smithsonian Institution Press, City of Washington, 1984.
4. João Orestes Schneider Santos, Paul Edwin Potter, Nélson Joaquim Reis, Léo Afraneo Hartmann, Ian Robert Fletcher and Neal J. McNaughton. Age, source, and regional stratigraphy of the Roraima Supergroup and Roraima-like outliers in northern South America based on U-Pb geochronology. GSA Bulletin; March 2003; v. 115; no. 3; p. 331-348. Link
5. Roy W. McDiarmid and Maureen A. Donnelly. The Herpetofauna of the Guayana Highlands: Amphibians and Reptiles of the Lost World. University of Chicago Press, 2005.
6. The Making of the Tepuis Film: "The Living Edens: The Lost World".
7. Miguel Lentino, Diana Esclasans: Áreas Importantes Para La Conservación De Las Aves En Venezuela In: BirdLife International y Conservation International. Áreas Importantes para la Conservación de las Aves en los Andes Tropicales: sitios prioritarios para la conservación de la biodiversidad., Quito, Ecuador: BirdLife International (Serie de Conservación de BirdLife No. 14), S. 621-730, 2005.
8. Franco Urbani (2009) Venezuelan Cave Minerals: Second Review. Mineralogy 1 2009 ICS Proceedings. In 15th International Congress of Speleology. p 345.
9. Zhu Xuewen and Waltham Tony. Tiankeng: definition and description. Speleogenesis and Evolution of Karst Aquifers. 2006.

Sima Humboldt and Sima Martel are included in the following lists and articles:

Unique formation

At first look in "Google Earth" one might think that Teiq sinkhole is just a widening of the deep canyon of wadi – perennial stream. Such wadis in Oman often have shaped deep valleys and ravines.

Closer look though reveals – here is something unusual. This giant hole is not a part of the valley. It is the endpoint of TWO streams that meet in the deepest point of this sinkhole and… disappear underground.

In the surroundings of the Teiq sinkhole there are several more interesting structures – round collapse dolines (look a bit to north-east!). These are signs of large, collapsed underground voids. Such voids are created by underground streams, flowing through cave passages.

Giant size

The size of Teiq sinkhole is very impressive. Width of this structure in SSE-NNW direction is 1,000 m, in NNE-SSW direction – 750 m. Depth reaches 250 m, average depth is 175 – 200 m. Volume of sinkhole is 90 million m³ – in this respect it belongs to the largest sinkholes of the world (the giant Xiaozhai sinkhole in China though is larger).

Walls around this sinkhole are very steep. In southern side walls of the sinkhole have even 150 m high overhang.

Wadis – perennial streams – enter the sinkhole as very spectacular waterfalls. During heavier rain this should be very impressive, unusual place.

History

Sinkhole was brought to international attention by a team of Slovenian explorers and Sultan Qaboos University in 1997.

It is not clear how this giant pit was formed. Maybe this sinkhole was formed by the collapse of a giant cave room. It is possible that this cave room was not as large as a current sinkhole and that sinkhole was later enlarged by two streams that enter it.

Here is located also Teiq Cave – very impressive cave with six chimneys – openings in the ceiling. In earlier times this giant cave was used by local people as a shelter. Both cave and sinkhole can be visited by tourists.

References

1. Taiq Sinkhole. Lilian en Jan Schreurs Homepage. (excellent source of information about Oman – highly recommended!) Accessed on August 5, 2010

Teiq sinkhole is included in the following article:

Heavenly pits – tiankengs

Origin of term

Since recent times scientists have proposed a special name for especially large sinkholes – tiankeng. This could be translated from Chinese approximately as “heavenly pit” or “sky hole”.

Tiankeng is a giant sinkhole with vertical walls, at least 100 m deep and 100 m wide. Most often bottom of tiankeng is crossed by an underground river, which is open to daylight only here.

Such structures are found in several countries (Papua New Guinea, Malaysia, Mexico) and, for example, in Mexico, such giant holes are called – sótano. But out of some 75 giant sinkholes of the world some 50, including the largest ones, are located in China. Thus to a large extent, this is truly a Chinese term for the Chinese phenomenon.

First tiankengs were documented in China in the early 1980s. The term “tiankeng” was proposed in October 2001 by Zhu Xuewen, China Cave Study Association and Institute of Karst Geology in Guilin.

How tiankengs are formed?

Nearly all tiankengs have developed in carbonate rocks with the noble exceptions of Sima Humboldt and Sima Martel in Venezuela, which have formed in sandstone.

Like other sinkholes, tiankengs are formed by karst processes (in carbonate rock) or suffosion (in sandstone). In karst processes rock is dissolved and carried away by flowing water, suffosion is a similar process, where particles of sandstone or similar rock are carried away by water.

Formation of tiankengs though requires very specific conditions, and a combination of these factors is very rare in nature. Thus – the correct recipe for a fine tiankeng looks like this:

• There is needed extremely thick and very clean layer of carbonate (or sandstone) rock. The thickness of the rock should be at least 200 – 300 m. Any layer of impurities will distort the process of formation.
• The layer of carbonate should be above sea level. It is even better if the area is raised by tectonic processes. Thus, for example, the Nakanai mountains in New Britain (Papua New Guinea) rise by some 3 mm in a year, also in China, the areas with tiankengs are rising.
• There is needed lots of rain. In areas with carbonate rocks, this inevitably leads to the formation of underground rivers. If the limestone layer is raised high, these rivers are flowing several hundred of meters below the surface.

If this is in place, nature does its work and tiankeng is formed.

In most cases there form collapse tiankengs – here underground rivers form giant underground rooms which at some moment collapse until the former cave room is turned into a vertical pit. The water of a powerful underground river dissolves the rock debris and carries it away.

There are known cases when erosion tiankengs develop – like Houping Tiankeng cluster (Chongqing Municipality). These giant holes are created by the source of underground rivers – here small streams fall through the many hundred meters thick limestone layers until they reach the impermeable rock. Then these streams continue like underground rivers. Gradually at this entrance is formed a hole which then increases up to incredible proportions. Nowadays such erosion tiankengs have beautiful waterfalls falling in them.

Tiankengs seem to be more short-lived than most other geological features. Initially, tiankeng forms as a smaller hole above the giant cave collapse, then this hole widens until the walls are vertical. The walls gradually erode and the former tiankeng turns into a forest-covered, enormous bowl-shaped depression.

Tiankengs belong to the most spectacular geological formations of the world and often house important biological values.

Largest sinkhole in the world – Xiaozhai tiankeng

Xiaozhai tiankeng was discovered by specialists in 1994 during the search for a new exploration site for British cavers in the China Caves Project.

In fact, it was well known to local people since ancient times. Next to this sinkhole was a small village – Xiaozhai. The name of the village in translation just means “little village”. Nowadays it is abandoned.

Already in Chinese classical works frequently are mentioned mysterious and deep ravines – possibly they describe tiankengs.

Xiaozhai tiankeng has incredible size. This hole is 626 m long and 537 m wide. The total depth is 511 – 662 m. The area of the opening is 274,000 m², volume – 119.348 million m³. All these millions of cubic meters have been dissolved and carried away by the river.

The sinkhole is a double nested structure – the upper bowl is 320 m deep, the lower – 342 m deep, and 257 – 268 m across. Between both these steps is a sloping ledge. This ledge has formed due to a muddy, “dirty” layer of limestone.

In wet weather, there is seen a waterfall falling into tiankeng.

This is one of three very large tiankengs worldwide – the other two are Dashiwei tiankeng and Haolong tiankeng (both in China).

Underground river and cave

Tiankeng has formed over the Difeng cave, which in turn has been formed by a powerful underground river. This river still crosses the bottom of tiankeng.

The underground river starts in Tianjing (Tiangjingxia) fissure gorge and reaches the daylight in the vertical cliff above the Migong River, falling like a waterfall 46 m high. The whole length of this underground river is approximately 8.5 km and during these 8.5 kilometers, it falls 364 meters. The average annual flow of the river is 8.77 m³ per second, but the flow rate can reach even 174 m³/s. This steep, powerful river has created the world’s largest sinkhole.

River and Difeng Cave have been explored and mapped in 4 km length by China Caves Project in 1994.

Location and geology

Tiankeng is located in a 280 km² large karst area, formed by lower Triassic limestone, Jialingjiang formation. The limestone here is extremely pure and even separate layers are not visible: it is just a monolithic, very thick block of pure limestone.

This tiankeng most likely formed over the last 128,000 years. For the most part of this time, it was an underground void, which gradually increased. The ceiling of this giant cave room collapsed several tens of thousands of years ago.

Not too far there are six more tiankengs – all other tiankengs in this area seem to be older than Xiaozhai.

Unique biotope

The bottom of this sinkhole is large enough to sustain a small, isolated world full of life. Here have been registered 1,285 species of plants, including such rare plants as ginkgo (Ginkgo biloba L.) and many rare animals like a clouded leopard (Neofelis nebulosa Griffith, 1828) and many others.

Tourism

Xiaozhai tiankeng belongs to a league of the most magnificent natural attractions of the world. We can be sure that it will be the desired destination for countless foreign tourists as well.

Domestic tourism in China is flourishing and starts to bring good profit to rural people. Thus any exciting attraction here turns into real economic value.

Earlier Xiaozhai tiankeng was nearly inaccessible, but the incredibly fast development of China over the last decade has brought many new roads to this area. Here has been developed also a zig-zag pathway with 2,800 steps descending into the tiankeng.

References

1. Zhu Xuewen, Chen Weihai, Tiankengs in the karst of China. Speleogenesis and Evolution of Karst Aquifers. The Online Scientific Journal. Accessed on the 31st of July 2010.
2. Zhu Xuewen, Tony Waltham, Tiankeng: definition and description. Speleogenesis and Evolution of Karst Aquifers. The Online Scientific Journal. Accessed on the 31st of July 2010.

Xiaozhai tiankeng is included in the following lists and articles:

The incredible Nakanaï Mountains

The exotic island of New Britain has many natural wonders – volcanoes, geysers, and numerous endemic animals, and plants. But unsurpassed wonders hide in Nakanaï Mountains – up to 2,185 m high mountains in the central-eastern part of the island. The total area of these mountains is 5,500 km².

Heavy rain

This place has an equatorial climate and the upper reaches of mountains belong to the wettest places in the world with yearly rainfall over 6,000 mm and reaching 11,900 mm.

This very frequent heavy rain and equatorial climate cause several interesting effects. One is easily visible – Nakanai Mountains are covered with dense, wild rainforests, considered to be one of the most biodiverse ecosystems in the world, with a large number of endemic species of plants and animals. Even nearly vertical cliff faces here are covered with dense jungle.

Karst

Another effect is even more interesting: Nakanai Mountains are made up of very thick (up to 1.5 km) layers of Miocene limestone, covered with a fragmentary layer of fine volcanic sediments from Pliocene. Perfect conditions for the development of karst processes!

Heavy rain creates rivers and rivers in the mountains over time create deep valleys. Throughout the last 200,000 years in the Nakanai Mountains have formed eight giant canyons with a depth reaching 600 – 1,000 m. The upper reaches of these canyons and their side ravines contain some of the most powerful springs in the world. So – these by far are not common rivers: the upper parts of these rivers are located deep inside the limestone layer, in giant caves. It is possible that these are the most powerful underground rivers in the world.

In some places in these caves formed enormous voids, which after some time collapsed (and many more for sure are going to collapse in the future). Now, these collapsed voids look like giant holes with underground rivers deep below crossing their bottoms – they can be compared to keyholes with a small fragment of the underworld visible through them.

In Nakanai mountains have been found numerous such sinkholes (also called collapse dolines and – if they are REALLY large and with vertical walls – tiankengs) – some have just opened and represent a narrow opening above giant underground voids. Others represent giant wells with vertical walls and others have degraded – they just resemble enormous, jungle-covered depressions.

Sinkholes of Nakanai Mountains belong to the most impressive, "best" sinkholes of the world. It is a bit hard to select – but it is possible that the most impressive among them is the Minyé sinkhole, although there are comparable ones, like Naré sinkhole.

Sinkhole

Minyé sinkhole looks like a giant hole in the green jungle, visible even in comparatively blurry satellite images. This hole is nearly perfectly round, with a 350 m diameter. The depth of the sinkhole is 400 – 510 m – the Eiffel tower here would be far below! The measured height has that much difference because the sinkhole is located on the slope – the upper rim is approximately 100 m higher than the lower rim.

Area of sinkhole is approximately 75,000 m², volume – 26 million m³. Equatorial climate helps to sustain jungle cover everywhere – on the bottom and even on the nearly vertical walls.

Floor of the sinkhole is crossed by a powerful river – from the upper rim it may look like a narrow stream, but it isn’t. The volume of the stream is 15 – 25 m³ per second. Doesn’t tell much? This would be enough to supply the needs of some 14 – 16 million Europeans. After torrential rain, the flow may increase significantly.

Cave

Initially it was considered that the cave passages at the bottom of Minyé are not too significant. This was not true.

Thus far there have been investigated 5,421 m long passages, but this may be just a small part of the system.

Powerful subterranean rivers require large passages – and caves under Minyé certainly are giants in this respect. The largest room in this cave is Tuké chamber – one of the largest cave chambers in the world. It is 240 m long, 200 m wide, and 180 m high. The floor area of this chamber is 48,000 m², volume is 6,240,000 m³.

Cave system contains exciting cave organisms, many of them endemic.

Exploration

Minyé was known to locals long ago.

Minyé, just like some other geological miracles of the Nakanai Mountains, were first researched by Australian speleologists in 1968 who descended to 60 m depth. Then French speleologists researched it in 1978 (first descent to bottom) and 1984 (French/ Swiss expedition), numerous expeditions came later as well.

Caves in Nakanai Mountains belong to the most exciting caves in the world but they require a lot – high skills in cave alpinism and cave diving, flawless health, and last but not least – a fat purse.

Caves in Nakanai belong to the technically most difficult caves in the world. Everything here is challenging – coming to this far-away (when looking from Europe or the United States) place, ascending the wild, jungle-covered mountains along the slot canyons of torrential rivers in nearly constant rain. Imagine building your camp in the wet jungle, in constant rain. Jeez, think about the bad odor from feet, better have separate tents… Then one should descend half a kilometer deep, through the jungle – can you imagine hanging in half a kilometer-long rope?

And only then it begins… Pristine, incredibly beautiful and opulent dripstone formations, giant passages, and the constant roar of rivers struggling through the cave and falling over countless waterfalls. Only the most experienced explorers should dare to dive into this powerful stream to pass the numerous siphons.

References

1. Waltham T. Tiankengs of the world, outside China, Speleogenesis.info. Accessed 24.07.10.
2. Vuvu 2010, speleological expedition. Accessed 24.07.10.

Minyé sinkhole is included in the following list:

History of exploration

Jaques-Yves Cousteau

Coast of Belize (and Mexico) is adorned with several elongated atolls. Further away from the mainland is located one of them – the Lighthouse Reef Atoll. Great Blue Hole is located in the central part of this atoll.

First explorations of this unique structure were made by legendary Jaques-Yves Cousteau in 1971. He and his team from ship Calypso charted the depth of the hole in 1971. They found that this is a karst formation which formed in at least four stages. They were excited about stalactites seen deep below the sea level – dripstone forms only above the water level. Cousteau noticed that stalactites were tilted – at the time when they formed, Blue Hole was in a different angle than now.

These exciting discoveries were shown in the very popular TV series “The Undersea World of Jaques-Yves Cousteau”, turning the Great Blue Hole into a true celebrity. Cousteau announced that this is one of the premium diving sites in the world.

Anthony Jones and Robert Dill

In 1997 Californian scientists Jones and Dill organized an expedition to explore the past of the Earth here.

Using the latest developments in diving gear, members of the expedition managed to reach the bottom of the sinkhole as well as to explore the side passages. Special attention was given to the exploration of stalactites. Isotope analysis of obtained pieces of stalactites provided valuable data about the past of Great Blue Hole.

Explorers reached and explored another exciting feature: Abyss in the northern part of Lighthouse Reef Atoll. This narrow fissure turned out to be at least 152 m deep.

Diving site

After a recommendation of such an experienced man as Cousteau Great Blue Hole turned into a popular diving site. As there is no dry land of significant size at Great Blue Hole, the diving trip here requires a full-day trip with local operators.

Divers though often don’t meet their expectations – while they expect to see countless weird sea organisms in such an extraordinary location, they see just an incredibly large hole with occasional larger fish, including numerous sharks. The true beauty here lies in this unique geological formation itself and the numerous riddles and answers about the past of Earth hidden here.

Rather late here was created marine protected area and around this same time in 1996 Great Blue Hole was included in UNESCO World Heritage Site list as a part of monument “Belize Barrier Reef Reserve System”.

Description

Size and shape

Great Blue Hole is a nearly perfectly round sinkhole, flanked by a crest of coral reef which rises above the sea in low tide. The diameter of the sinkhole is approximately 318 m.

This sinkhole is a true blue hole that stands out against the light turquoise blue lagoon around it. Such formations are possible only in large carbonate platforms covered by the sea.

Depth as measured by the team of Cousteau was 125 m. Jones and Dill in 1997 could not find a location deeper than 124 m – whether due to measurement imprecision of the previous team or due to the sedimentation over 26 years long period.

There are some reasons to believe in such rapid sedimentation processes: there are no waves or currents below the depth of 18 – 20 m in Blue Hole and thus the sinkhole serves as a sediment trap. There exist similar structures which are filled with sediments – for example near Key Largo in the Florida Keys (United States).

Sinkhole has an hourglass shape in cross-section.

Hydrogen sulphide layer

Unusual feature of Great Blue Hole is turbid, a brownish layer of water at the depth of 90 – 101 m, enriched with hydrogen sulfide H₂S. Water above it and below it is lucid – with the difference that below this layer the water does not contain oxygen. The bottom of the Great Blue Hole is lifeless, covered with large pieces of limestone.

This unusual hydrogen sulfide layer has been created by the constant pouring of organic substances in the Great Blue Hole. As these pieces of organic matter sink, they are decomposed by microorganisms. These microorganisms deplete all the oxygen at the bottom layer of water and form hydrogen sulfide. This, in turn, makes the water more acidic and dissolves the limestone walls. Most likely this process has helped to create the hourglass shape of the sinkhole.

West Wall Cave

In 1997 at the 50 m depth in the west wall of Blue Hole there was discovered a cave – West Wall Cave. After this deep entrance, the cave rises up to 37 – 41 m depth and then extends horizontally for more than 46 m. The cave contains many skeletons of marine turtles and other creatures. The depth of this cave coincides with the depth of the majority of stalactites and stalagmites.

Stalactites and stalagmites

Most exciting finds of Cousteau were stalactites. Exactly these stalactites lured also the expedition of 1997.

Most stalactites are located at the depth of 30 – 45 meters. They can reach an impressive size – some are even 8 meters long. Divers found also several stalagmites.

Data record of the past

Dripstone formations need to be in the atmosphere – above sea level. This means that at some time in the past at least the upper part of Great Blue Hole was above sea level. Even more – whole depth of sinkhole needs to be above the water level – formation stops at the water level (there exist exceptions in volcanic regions – see El Zacatón sinkhole in Mexico.).

Stalactites themselves can provide an answer about the age of their formation – through the method of isotope analysis. The team of the expedition from 1997 took numerous samples (one of the rare cases when damage to stalactites is justified) and analyzed them.

Result was exciting and at the same time foreseeable – stalactites (and the Blue Hole itself) have formed during at least four episodes of low sea level – 153,000, 66,000, 60,000, and 15,000 years ago. This corresponds well to other data of climate changes in the world, showing lower temperatures at these periods.

Tilt

Another exciting feature of stalactites and stalagmites is their tilt. Some are straight, some tilted, some are half tilted and half straight. These dripstone formations have fixed some tectonic change in the past when the angle of the Great Blue Hole and surrounding layers of carbonate changed.

Tilt of dripstone formations is some 5° – less than 11 – 13° reported by the expedition of Cousteau.

Reports of Cousteau team about stalactites at 120-meter depth are not credible either – acidic water at this depth would dissolve any dripstone formations.

Unclear origins of Great Blue Hole

The mechanism of formation of the Great Blue Hole remains unclear. First – it is too deep if we remember that its full depth is a lot larger and the true bottom is hidden below a thick layer of sediments. Sea level in the past did not fall THAT much.

Second – it is not clear how the sinkhole formed. Lighthouse Reef Atoll is small and separated from the mainland by at least 1.200 m deep sea. Sinkholes require a rather powerful flow of freshwater – and consequently, there is needed sizeable landmass.

References

1. A.T.Jones and R.F.Dill, The Great Blue Hole of Lighthouse Reef Atoll, Belize, Central America: Deep Technical Diving to Collect Sea-Level Records, 181 – 195 pp. Caribbean Geology Into the Third Millenium. Transactions of the Fifteenth Caribbean Geological Conference. Edited by Trevor A. Jackson. 2002. Accessed on July 19, 2010

Great Blue Hole is included in the following article:

Dinaric karst

Dinaric mountains are formed of unusually thick and clean carbonate rock layers – up to 7 km thick. Tectonic processes in Paleocene – Eocene (some 50 million years ago) started to fold these limestone layers into Dinaric Alps which we know today. This process of folding still continues.

During the folding the limestone was broken and in fissures there developed caves and other karst formations.

In total in Dinars are known more than 8,000 landmarks created by karst processes such as caves, sinkholes, powerful springs.

Sinkholes of Imotski

Around the Imotski town near the border with Bosnia-Herzegovina in the upper Cretaceous limestone there have developed unusual, extremely large sinkholes – in total some 20 giant holes, formed by an underground river flowing on the contact of limestone (above) and dolomite (below it). The town itself developed at one of such sinkholes – the 220 – 290 m deep Modro Jezero.

The largest of sinkholes near Imotski is Crveno Jezero – the largest karst landform in the Dinaric mountains and one of the largest sinkholes in the whole world.

General description

The name of the sinkhole – Red Lake – stems from the color of surrounding cliffs, which contrast with the bright blue color of the lake. These walls are reddish-brown, colored by iron oxides of Mediterranean soil Terra Rossa.

Sinkhole is roughly circular, with nearly vertical, extremely tall walls and the deep blue lake far below. Walls are nearly vertical from three sides and slanted from the fourth. This fourth side collapsed during the earthquake in 1942.

Color of the lake changes from green to dark blue, depending on the sky above it.

Limestone walls rise over the average level of this lake 240 – 260 m tall, dwarfing the forest which grows around it.

Total known depth of the sinkhole is approximately 530 m. Width of sinkhole – 450 – 500 m.

Volume of this sinkhole (not a lake but a whole sinkhole) is estimated to be 25 – 30 million m³, the volume of the lake is at least 7 million m³.

Diameter of the lake is some 200 m. The known depth of the lake is 280 – 290 m. Data about the depth of the lake are somewhat controversial. This can be easily explained – the level of the lake may change per 30 – 50 m.

Level of the lake can change even for 1 m in a day – that requires a flow of hundreds of millions of liters per day.

The bottom of this lake is located at least 5 meters below the level of the sea. Thus the original cave, which formed this sinkhole, formed during the ice ages, when the sea level was lower.

When the level of the lake is lower, it is not available – cliffs over the last 30 m are overhanging all around the lake. Though – if the level of the lake rises, it becomes available – but also after a hard and risky climb.

Bathing in the lake water, for the most part, is really chilly – above it may warm up – but not too much as the Sun reaches it for a too-short time. Deeper the temperature of water decreases to 7 – 8 °C, water is very very clean and lucid.

Crveno Jezero most likely has been formed by the collapse of an enormous cave hall. It seems to be the youngest of Imotski sinkholes – due to its vertical walls. But maybe it simply has a more active underground river which does not let to accumulate sediments.

Smaller caves

Near the water level of the lake there have formed smaller caves. One of such caves – Czepitschka Cave – has been explored in 1998. It has one entrance above the water level and another – below it. The total length of this cave is 670 m.

Some other caves are Kik-kak (some 15 m long), Balkon Prijatelstva. These caves have formed by the intermittent level of water in the lake.

Exploration

Crveno Jezero, of course, is known since ancient times. But Wondermondo knows about just one serious exploration expedition to this sinkhole (maybe there have been more). This expedition was organized in 1998 by German, Austrian and Croatian speleologists and scientists, led by Aspacher, Behrend, Haslinger, Hilbert. This was a complex expedition, requiring very high expertise and experience in climbing and deep diving.

In hard work there was obtained valuable information. For example, explorers met a powerful underwater stream at 170 m depth. Thus most likely this sinkhole is crossed by a powerful underground stream. This explains also why there was not noticed a layer of debris on the floor of the lake.

In fact, one of the highest dangers for any explorer here is posed by the falling cliffs. On their way back to the surface explorers learned how dangerous it may become: during the thunderstorm collapsed enormous cliffs and after several hundred meters high free-fall splashed in the lake. Even in calm weather, occasional stones fall in the lake.

Mysterious fish

Crveno Jezero contains several organisms who live only here and nowhere else in the world.

One of them is small, up to 12 cm long fish – Delminichthys adspersus (Heckel, 1843) (earlier Latin names of this fish are Leucos adspersus, Paraphoxinus adspersus).

In fact, this fish occasionally, during dry winters and springs, can be seen in springs in the region around the sinkhole – it then spreads even in the surrounding rivers and lakes. This means that Crveno Jezero has underground connections to these springs.

This fish is known to science since 1840 – but only in 1998 did explorers to see it in its natural environment.

The lake contains lots of this fish. Even at 70 m depth there have been observed large swarms of Delminichthys adspersus – with up to 200 fishes in such groups. In the evening they rise up to the surface of the water and are seen from above as "mysterious" black spots.

In Dinaric Alps there are found several more related species of Delminichthys genera – but none outside Dinaric Alps. These fishes require extremely clean water from Dinaric karst and are highly endangered.

References

1. PATD: Bernd Aspacher, Ralf Haslinger, Heinz Knauseder, Ralph Wilhelm, Beyond the Blue. Ein Vorstoß in unbekannte Tiefen. Accessed on August 13, 2010

Crveno Jezero is included in the following list:

Giant hole

Sotano de las Golondrinas offers one of the deepest freefall drops in the world. The entrance of this sinkhole is elliptical in shape, 49 by 62 m wide. Deeper this natural wonder becomes much wider – at the bottom, it is 303 by 134 m large (some sources state 305 x 440 m). The area of the bottom can be compared to three football fields.

Depth of the entrance shaft, when measured from the lower rim, is 333 m, from the upper one – 372 m.

That’s not all – at the bottom of this entrance shaft, there is a narrow continuation that leads through several steps up to 512 m depth. But few people go there – this is not the main attraction here. This smaller shaft goes directly down. Thus it is possible that the bottom of the sinkhole is covered with a heap of rubble and without it Sotano de las Golondrinas would be more than 500 m deep.

These were pure figures. If frankly – they don’t tell much. Let’s try it otherwise – if you would jump in this hole, you would fall down for 12 seconds. Count it – that’s a lot!

In rainy season there is added effect – several smaller and larger waterfalls are falling directly inside the cave, in one incredible plunge. These certainly are some of the tallest cave waterfalls in the world.

At certain temperatures and dampness there form clouds at the upper part of the cave. Yes, this cave is large enough to have true clouds inside!

Karst formation

This incredible sinkhole has been shaped by water in a fault of impermeable limestone plain. It has formed in lower Cretaceous limestone and is located in the approximately 500 km long and up to 3,500 m high Sierra Madre Oriental mountains.

Sotano de las Golondrinas by far is not the only karst formation here. Here, around Aquismón town, are located several very deep sinkholes and some very long cave systems including the giant Sistema Purificación, explored to 95 km length and up to 957 m depth. There is even larger sinkhole by volume – Hoya de las Guaguas with 202 m deep entrance shaft and total depth of 430 m. Sótano del Barro 67 km to the east offers even deeper freefall drop – 410 m!

Birds

At sunrise, around 6:00, here happen incredible things. Observers at the rim of the sinkhole at first notice small, light grey spots slowly circling far below. Gradually these spots become larger and larger – these are thousands of white-collared swifts (Streptoprocne zonaris Shaw, 1796) rising up from the sinkhole. Birds are moving simultaneously, in a well-studied primeval movement. This spiraling ascent leaves a dizzying and even hypnotic effect on observers. As the birds reach the surface their flight becomes chaotic and erratic – here are awaiting numerous birds of prey. Soon, in a few minutes, all swifts disappear until the evening.

This event continues for some 20 – 30 minutes. It takes place every morning and most likely – for thousands of years. In the evening birds gather above the cave and then possibly quickly, with folded wings and whistle fall down – thousands and thousands of desperate grey birds escaping their predators…

Birds have good reason to live here – nests on the overhanging walls are safe from predators.

Swifts have given a name to the sinkhole – “Sotano de las Golondrinas” in Spanish means “Cellar of the Swallows”.

But here lives another, more spectacular bird. These are the green parakeets (Aratinga holochlora Sclater, 1859) – beautiful green parrots. Also, these birds rise up from the cave flying in spirals. They appear as lively green spots far below.

Guano and fungi

Bottom of the sinkhole is unexpectedly large. It is covered with a thick layer of guano with emerald green moss on it. Water has shaped here small canyons up to 2 m deep. This is a very unusual, chilly, surprising small world. Some people build tents here and in the next morning experience a very surrealistic sunrise.

But stay here is not very pleasant. On the bottom live millipedes, insects, snakes, scorpions. The air is filled with smells, fungi, and bacteria. One can easily get ill with histoplasmosis – a lung disease caused by fungi in guano. The use of a respiratory mask is advised to those who want to inspect all the corners of the sinkhole.

Centre of adventure tourism

Sinkhole is located in a mountainous area, in a tropical forest. This area since ancient times is inhabited by Huastec people and most certainly the enormous hole in the ground is known for thousands of years. Not far from the sinkhole, across the valley, there have been found interesting rock paintings of the procession of warriors led by a young woman.

First known attempt to investigate the sinkhole was taken by Texas cavers T.R.Evans, Charles Borland, and Randy Sterns on December 27, 1966. In 1969 there was discovered the continuation of the cave up to the depth of 512 m.

Back then the access to the sinkhole took several hours – this was an exotic but hard experience.

Now the times have changed – the road almost reaches the sinkhole, the walk takes only some 20 minutes. The extreme and fascinating sights draw many tourists to this natural wonder. The sinkhole is used by many adventure tourists – many are rappelling down and some make BASE jumping inside this incredible hole. Once the bottom of the cave was reached even by a hot air balloon.

The nearby Aquismón town has turned into a small center of tourism and numerous local people have found a new source of income without leaving their home country.

But this natural wonder is also a victim of its own popularity – ornithologists have noticed a decrease in numbers of bird population – possibly due to disturbance of cavers and other tourists.

In recent years there have been introduced limitations. One of such limitations: the descent into the sinkhole is made only from one and the same spot. The descent should be made in the daytime so that birds are not disturbed.

Of course, it is forbidden to leave or throw any things in the hole. It is also forbidden to use helicopters near it and to shout in the hole for an echo. Visiting the sinkhole is not for free anymore.

Descent into abyss

The impression from rappeling down into this sinkhole exceeds any expectations. No human can be prepared to experience such sight. Descent into this giant hole might seem too slow and then – when the floor of the cave comes closer – it turns out that descent is faster than it seemed before. Metal parts of rappelling equipment become very hot and should be cooled down by sprinkling bottled water on it – otherwise, the metal can melt the rope. One should not stop midway too – this might create the overload on equipment.

The way back often is made with specific American rope climbing technology – "ropewalker" – then it takes only some 30 minutes. When going up, the rope is stretching and swinging due to extreme length – this may cause even motion sickness.

Sótano de las Golondrinas is included in the following lists and articles:

Black holes of Bahamas

Phenomenon of black holes is a recent discovery and thus far they are known only in Andros and Grand Bahama islands in the Bahamas.

Only in 1985 from the airplanes in the South Andros Island there were noticed round holes filled with abnormally black water. Initially, these were considered to be meteorite craters.

Although this region is close to densely populated and affluent areas with numerous scientists and caving enthusiasts, only in late June 1999 there took place the first scientific expedition to the largest and most impressive of these holes – South Andros Black Hole. Only then it was discovered that these round, black lakes are unique, unprecedented natural formations.

Giant layer of carbonates

The Bahamas are the visible part of a giant "heap" of carbonate rock. For some 200 million years, since Jurassic times here have accrued limestone, aragonite, dolomite, and gypsum layers with total thickness exceeding 6 kilometers.

Interaction between the porous carbonate rocks and seawater has created countless, extensive cave systems, most of them still to be discovered and explored.

More common here are blue holes – beautiful, diverse, deep caves filled with seawater and lined with bright white carbonate sand. Light refraction here creates a crystal clear, beautiful blue color.

Formation of black holes

Today black holes are located in shallow, muddy flats. The layer of carbonate mud can be several meters deep, making these places unavailable by foot or boat. Low-lying mudflats have numerous inundated areas with shallow seawater and rainwater poodles, bays, and lakes.

Most likely similar conditions have lasted here for a long time. Sea level though has changed often – local mudflats over the last 1.8 million years some 30 times have experienced ascent and descent of sea.

At some locations the aggressive seawater started to dissolve the carbonate rock at the base of poodles, gradually forming round, rather deep holes. Thus, contrary to many sinkholes around the world, black holes have not formed from deeper layers to the top, but in the opposite direction – from the surface downwards. Most likely they are not linked to the sea and they do not have lateral passages.

Location and number of black holes

In total there are more than 30 black holes in the Bahamas. All of them except for one, are located on the largest island – Andros, especially in the southwestern and central part. One black hole is found in Grand Bahama.

Location of most black holes is not popularised – these unique monuments of nature easily could be endangered by random visitors, especially – by diving.

First dive in the black hole

In late June 1999 the most impressive black hole – the Black Hole of South Andros – was reached by a team of scientists and movie makers. Group was led by the experienced cave diver, lawyer, and geomicrobiologist Stephanie Schwabe.

Most likely this was the first time when humans reached this weird lake. Low-lying banks of Black Hole contained black, uninviting water.

There were made several tests throughout the whole depth of the lake, measuring several values. Turned out that the lake is layered with the oxygen-depleted layer below.

Nonetheless the first dive brought a great surprise. The dark waters turned out to be crystal clear and at the depth of more than 17 meters, there was visible something like a weird, muddy bottom.

As Stephie Schwabe lowered her hand in this mud, she jerked it back in surprise – it was hot! The disturbed surface of this fake bottom undulated like a jelly.

Schwabe just dived into this weird jelly (would you do this?). The hot, dark pink substance suddenly ended, and the diver reached the next layer – cool, crystal-clear water in total, absolute darkness.

Further descent continued into complete darkness until reaching the bottom. Return back to the surface, of course, required repeated passage through the unpleasant, hot jelly.

When divers returned back to the daylight, they gazed in surprise at all the metal parts of their equipment – everything, even a silver bracelet, was turned black.

Description of the Black Hole

The Black Hole of South Andros is a nearly perfectly round lake with a diameter of 270 – 310 m.

Thus far its measured maximum depth is 47 meters. In the southern part of the lake, the full depth is reached suddenly, and then, northwards the bottom gradually rises upwards. The banks of the lake are low.

Lake has three distinct layers of water.

Upper layer

The upper layer reaches a depth of 17.7 m. It is a fairly common waterbody – it consists of clear, lucid brackish water with 12 ‰ salinity. Water is very translucent, and even at the depth of 17.7 m, there is enough light.

Temperature of the water is common for these areas – 26° C. In this layer live some fish and zooplankton. The walls of the lake in this layer are covered with up to 0.5 m thick cover of algae.

Bacterial layer

At the depth of 17.7 – 19 meters there is located the most exotic feature of black holes – the bacterial layer. Similar layers have been found also in other black holes of the Bahamas, including the nearby Jacuzzi Black Hole.

This approximately 1 m thick layer contains a huge amount of phototrophic purple sulfur bacteria. These organisms basically need sulfur and light for their existence.

Elsewhere in the world there also is met purple sulfur bacteria and during the bloom, their number might become significant too. But these bacteria, in general, are located at shallower depths: 5 – 10 m. Perhaps in Andros, the water is very translucent and bacteria have the best circumstances at greater depth here.

Thus far in Andros Black Hole have been found several species of bacteria, belonging to Chromatiaceae family, Thiocapsa and Allochromatium genera. One endemic species – Allocromatium palmerii – has been described thus far.

Estimates show that the Black Hole of Andros contains some 5.6 tons of dry-weight of bacteria. The density of bacteria in a layer is more than 10 million living cells in a milliliter.

Water in the bacterial layer is acidic and contains more sulfides – while at the surface adicity is 8.6 pH, here it is 6.5 pH. The layer of microorganisms has left a trace – etching in the walls of the lake. This acidity also explains why the metal details turn here black.

The most unusual feature though is the rise of temperature. In the Bahamas, there is not met any volcanic heat and no geothermal water. The only explanation is that the heat has been created by microorganisms themselves.

Temperature of water here sharply increases from 29 °C to 36°C, and might reach even 40°C. At the 20 m depth, it returns back to 29 ° C.

Most likely this is the only known case in the world where microorganisms in nature significantly increase the temperature of the water. This heated water, in turn, provides a competitive advantage over other species that are not thermoresistant.

Heat is created by chemical reactions in cells where the excess light energy is turned into heat. Further increase in temperature is not expected – the amount of microorganisms here seems to be limited by a lack of sulfur.

Another factor helping to sustain this heated bacterial layer is extremely stable conditions in the lake. It is well possible that Stephi Schwabe was the first nuisance in this eternal bacterial stillness. We can be sure that many divers crisscrossing this layer would rather soon disrupt the balance.

Thanks to bacteria black holes are black. The microbial layer virtually “eats up” the light and no light is reflected from this layer.

Bottom layer

Below the bacterial layer water again becomes cooler (26° C) and lucid.

But there is a large difference from the surface water. The bacterial layer absorbs all the light efficiently, and there is total darkness. The water here does not contain any oxygen either but it is saline – with 35 ‰ salinity.

Floor of the black hole is covered with a thick layer of purple-orange microbial mats.

Bottom layer is little explored but there are specialists who consider that it in many respects resembles the ancient oceans of the Earth some 3.5 billion years ago, when there was nearly no oxygen in the atmosphere and water.

Dangerous place

Diving in the Black Hole of South Andros is not allowed, except for specific scientific purposes. The same refers also to other black holes in the Bahamas.

Here should be mentioned that such diving would be exceptionally dangerous – the bacterial layer is very toxic, hot and it easily damages the diving equipment. Below it there is complete darkness and the inexperienced diver can easily get lost here. The depth here is large enough to be dangerous.

But even coming here is very complex – black holes can be accessed only by helicopter.

Thus – enjoy reading about this weird place, you have almost no chance to see this bacterial layer. And, if frankly, it doesn’t need your presence.

References

1. Stephanie Schwabe, Rodney A. Herbert. Black Holes of Bahamas: what they are and why they are black. Science Direct.
2. Stephanie Schwabe, The Floor of Fire, Divernet.

Black Hole of Andros is included in the following lists and articles:

Sistema Zacatón

El Zacaton sinkhole belongs to unique karst field – Sistema Zacatón. These karst features have incredible characteristics without parallels elsewhere in the world. In total, this area contains some 20 unusual features – sinkholes, caves, springs created by deep groundwater heated by volcanic processes. Karstification processes here continue today as well – acidic groundwater continues to strip away the limestone and deposits it in unusual travertine formations.

El Zacatón sinkhole is the most prominent and the best-known feature in this karst field. It is the deepest sinkhole in a group of five unusual sinkholes which are located close together but are not directly connected.

Dramatic exploration

Often people think that Zacatón was discovered only a few decades ago but this is not true.

We can be sure that indigenous people in this area knew about the sinkhole. Félix María Calleja, viceroy of New Spain, in 1795 wrote: "There is a large cave lit by natural skylight (most likely Las Quarteles) and 200 varas (some 170 m) from this cave there is a deep cavity that has a lake with an island.". We can be sure that he meant exactly El Zacatón.

Deep diving in El Zacaton

Attention of professional divers turned to this extremely deep sinkhole in the late 1980s and El Zacatón turned into a historically important dive site with several scuba diving world records set here.

One of the driving motives behind this interest was a wish to find the true depth of the sinkhole. As turned out later – the common measurement technique by using a measuring tape and weight is not applicable here – in 1990 such measurement gave a depth of 76 meters – weight stopped on a prominent ledge. In reality, though the sinkhole is several times deeper.

Descent into the lake along the vertical cliffs is very complicated. In 1990 there was discovered a more convenient way to enter El Zacatón: from the nearby El Nacimiento spring through a 230 m long and 17 m deep underwater cave passage called Passage de la Tortuga Muerta. Now this passage is the most often used passage to enter El Zacatón.

Women’s depth record in diving was reached here in 1993 by Dr. Ann Kristovich: she reached the depth of 169 m.

On April 6, 1994, there happened both a tragedy and an amazing achievement. Two very experienced cave divers – Sheck Exley and Jim Bowden simultaneously dived in the lake. Jim Bowden reached the world record – 282 m but Sheck Exley died due to unexplained reasons at the depth of 268 – 276 m. Both divers were not too far from each other but in the darkness could see only the lights of their partner and at such record depth, none of them was able to go and see how the other diver feels. The true depth of El Zacatón was not reached and still, there was no idea how deep it is.

Diving robots

These tragic events changed the strategy of further research. Researchers decided to used robotic equipment.

El Zacatón was selected as a suitable place for scientific experiments by NASA and several other institutions. This unusual place served as a testbed for a semi-autonomous robot called Deep Phreatic Thermal Explorer (DEPTHX).

DEPTHX robot was successful and reached the bottom of the sinkhole in late May 2007. DEPTX also mapped the deepest part of El Zacatón which has been unavailable to people.

At large depth the robot collected samples which led to the discovery of new species of bacteria.

In the future the knowledge obtained in this experiment may help to develop technologies for the exploration of the Europa – moon of Jupiter, which has wast salt ocean – a potential source of indigenous life. Thus El Zacatón has played a role in the development of space technologies.

Lots of warm water

El Zacatón is the deepest water-filled sinkhole in the world. The total depth of the sinkhole is 339 m, the lake in it is 319 m deep.

In Italy has been found even deeper water-filled sinkhole – the 472 m deep Pozzo del Merro with 392 m deep lake. Even deeper is Hranice Abyss in Czechia with its 404 m deep lake. These though cannot be considered to be true sinkholes – both are caves.

At the water level the diameter of El Zacatón is approximately 116 m but the dimensions of the lake increase with the depth.

Bottom of El Zacatón is some 130 m below the sea level – such seep sinkholes are not created by normal groundwater action but rather by volcanic processes.

Volume of the water in the sinkhole is 9.5 million m³. The temperature and chemical content of the water is homogeneous except for a thin level of rainwater on the surface.

Water of El Zacatón is 29 – 30° C warm. It is highly mineralized, with a sulfurous odor, slightly acidic with pH 6.9.

Color of water is not homogenous – at some locations, there are light clouds formed by the particles of precipitated sulfur.

Unique microorganisms

Like in several other sinkholes of Sistema Zacatón the travertine walls of the upper part of the sinkhole are entirely covered with purple, red, and green biomats extending from the surface to 35 – 40 m depth. Closer to the surface these mats are green colored but at some 3 m appear purple biomats. These microbial communities disappear totally below 100 m depth – it means that these microorganisms are dependant on the light.

But in greater depths, there live other microorganisms. There have been found at least 9 new classes of microbes on the rock walls at 115 – 270 m depth.

Floating islands

El Zacatón is one of the very few places in the world with floating islands formed from precipitated calcium carbonate. According to written sources, such islands were floating also in Italy near Tivoli, in sulfurous waters of Lago della Regina – nowadays they don’t exist.

These islands have given a name to the sinkhole – they are covered with tall grass called "zacate" in Mexican Spanish.

Islands are free-floating and are moved around the surface of the lake with the wind. They are roughly circular and with steep sides from regularly bouncing off the walls and each other. Diameter of islands is 3 – 10 m.

It is possible that these unique islands eventually will grow bigger and at some moment may cover the whole sinkhole – creating a travertine lid like in Poza Seca and several more sinkholes in Sistema Zacatón.

Such process of naturally created floating travertine islands is very rare in nature. It may be formed by the chemical processes in volcanic heated, slightly acidic water – as the water meets air, dissolved carbon dioxide outgasses, leaving particles of lime at the surface.

Created by volcanic heat?

Long-term researcher of El Zacatón – Marcus Gary – considers that the unusual karst landscape of Sistema Zacatón has been formed by deep volcanic activity. Volcanism heats the deep groundwater and turns it slightly acidic by adding dissolved carbon dioxide and hydrogen sulfide.

As this water reaches limestone layers, it gradually dissolves it, forming voids. Such a process is called "hypogenic karstification". As the void becomes larger, the limestone layers in the ceiling collapse one after another finally reaching the surface. If this theory is true, the formation of these sinkholes differs very much from the mechanism of formation of most other sinkholes.

References

1. Marcus Gary, Sistema Zacatón, research homepage. Accessed in the 23rd June 2010
2. Marcus Gary, El Zacatón, image. Accessed on June 27, 2010
3. Marcus O. Gary, John M. Sharp Jr.Volcanogenic karstification of Sistema Zacatón, Mexico. Accessed on July 2, 2010

El Zacatón sinkhole is included in the following lists and articles: