There exists a broad term "blue hole" describing any deep, water filled, vertical caves. The deepest known blue hole in the sea is the 202 - 203 m deep Dean's Blue Hole in Bahamas, Long Island.
The deepest seawater blue hole in the world
Dean's Blue Hole is named after the family name of local landowners. It is located in a small bay, divided from the open sea with small peninsula.
At the very tip of this small bay in the depth of 6 m starts entrance into this vertical cave. Entrance is 25 x 30 metres wide. Here starts the sinkhole - it's walls are not just vertical but overhanging. There can be observed a constant "sandfall" - small rivulets of sand are streaming down into the hole.
Further down the blue hole becomes larger and larger, until the diametre reaches approximately 100 m. It is one of the largest known underwater cave rooms in the world.
Sea water here is lucid and unusually calm - thus the visibility reaches up to 15 - 30 m depth.
Bottom of the cave is rather even. Walls though are not even, there are seen numerous unexplored side passages. Exploration of these passages is very complex due to the great depth.
During the tides enormous volume of water is discharging through the mouth of cave - it means that at least some of these cave connections are deep and connected to the sea.
Here live young tarpoons (large seafish), occasional fish of other species and turtles can be met.
Dean's Blue Hole sometimes erroneously is presented as the deepest water filled sinkhole in the world. Several water filled sinkholes though are deeper. For example, Pozzo Merro in Italy is 392 m deep and the giant El Zacatón sinkhole in Mexico is 339 m deep.
There is some huddle regarding the terms "sinkhole" and "blue hole". Some say that blue holes are not exactly sinkholes but rather vertical caves. The fact is that sinkholes ARE a kind of vertical caves - sinkholes are holes or natural deppressions caused by karst processes.
"Blue hole" is a term applied to some of those sinkholes which are filled with water - irrespective of their mechanism of formation. It seems that the decisive factor in the use of this term is the beautiful deep blue color of water in these formations.
This color is created by high transparency of water and bright white carbonate sand. Blue light is the most enduring part of spectrum: where other parts of spectrum - red, yellow and finally the green - are absorbed during their path through the water, blue light manages to reach the white sand and return back after the refraction.
Bahamas were the first where this term has been applied to diverse beautiful sinkholes filled with the sea water. Blue holes here (as elsewhere in the world) formed in times when the sea level was lower than now. Sometimes these holes were formed by deeper groundwater gradually dissolving the limestone until the ceiling of these voids collapsed. Sometimes though they were formed by the rainwater seeping into limestone through a crevice and gradually making it wider.
Later the sea level rised and these vertical holes were filled with lucid, calm water.
Most likely Dean's Blue Hole formed in a similar way some 15,000 years ago, though it is not clear why this sinkhole is that deep - sea level in Bahamas back then was a bit more than 100 metres belov the present sea level.
World record site
Here, in this beautiful geological formation are meeting the might and beauty of nature and hardiness of people.
Full depth of this sinkhole was first reached by Jim King in 1992.
Over the last few years this sinkhole though has become a scene of world-class free-diving. Since 2007 here is established a freediving school "Vertical Blue" (operates from November to May), which organises also yearly world competition in April.
This deep sinkhole is perfect for such sports - it is located right next to the bank, in calm water.
Cave has seen many incredible world records in diverse disciplines of free-diving by the best freedivers of the world, including Natalia Molchanova, Herbert Nitsch, William Trubridge.
One such discipline, for example, is - Constant Weight Apnea Without Fins (diving possibly deep and returning in one breath without any appliances and without holding at the rope). How deep one can go in such way? 10 metres? 15 metres? No - in 13th December 2010 William Trubridge dived 100 metres deep (and then 100 metres back) in such a way, it took 4 minutes 10 seconds.
Try at least to run 100 metres forth and 100 metres back without breathing - even that is a serious challenge!