North America, Mexico, San Luis Potosí, 45 km south from Ciudad Valles and 8.5 km west from Aquismón
Sotano de las Golondrinas, Pozo de las Golondrinas, Golondrinas collapse pit
Depth of sinkhole:
Volume of sinkhole:
5 million m³
Depth of cave:
One of the most impressive sinkholes in the world is Sótano de las Golondrinas - Cave of the Swallows. This hole in the ground is that large that it is nearly impossible to comprehend it - even if one is sitting at the rim of it.
Sótano de las Golondrinas offers one of the deepest freefall drop in the world. Entrance of this sinkhole is elliptical in shape, 49 by 62 m wide. When ascending 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 narrow continuation which 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 sinkhole is covered with heap of rubble and without it Sótano 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!
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.
Sótano 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 Guasguas 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!
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 well studied primeval movement: this spiralling ascent leaves dizzying and even hypnotic effect on observers. As the birds reach the surface their flight becomes chaotic and erratic - here are waiting 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 - since 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 - "Sótano de las Golondrinas" in Spanish means "Basement of the Swallows".
But here lives another, even 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 thick level of guano with emerald green moss on it. Water has shaped here small canyons up to 2 m deep. This is very unusual, chilly, surprising small world. Some people build tents here and in the next morning experience very surrealistic sunrise.
But stay here is not very pleasant. On the bottom live milipedes, insects, snakes, scorpions, air is filled with smells, fungi and bacteria. One can easily get ill with histoplasmosis - lung disease caused by fungi in guano. If you are interested to inspect all the corners of sinkhole, it is advised to use respiratory mask.
Centre of adventure tourism
Sinkhole is located in mountainous area, in 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 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 in 27th December 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 exotic but hard experience.
Now the times have changed - road almost reaches the sinkhole, walk takes only some 20 minutes. The extreme and fascinating sights draw many tourists to this natural wonder. Sinkhole is used by many adventure tourists - many are rapelling 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 small centre of tourism and numerous local people have found new source of income without leaving their home country.
But this natural wonder is also a victim of its own popularity - ornithologists have noticed 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. Descent should be made in 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 echo. Visit to the sinkhole is not for free anymore.
Descent into abyss
The impression from rappeling down into this sinkhole exceeds any expecations. 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 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.