|Location:||Near Clarence Town on Long Island, Bahamas.|
|Classification:||Karsthöhle Doline or sink|
|As far as we know this information was accurate when it was published (see years in brackets), but may have changed since then.
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Dean's Blue Hole is the world's deepest known blue hole.
Blue hole is a geographic term, which describes dark blue circular spots in the shallow Carribbean sea. They are caused by cave shafts aka potholes ending on the sea floor, which are the natural entrances of huge cave systems below. The caves formed during the cold periods of the Ice Ages, the glacials, when the sea level was 100m below todays sea level. As a result most of those blue holes are around 100m deep.
This blue hole is exceptional, as it is more than 200m deep. The opening is almost circular with a diameter between 25m and 35m. The walls of the hole are vertical, there are even some overhanging parts where the shaft widens. How this happened is not known, as the cave development normally happens in the upper area of the phreatic zone. This means a little below and above sea level if you are close to the sea. Probably there was a downlift of the whole island or probably there was underwater hypogene solution of limestone, probably because of mixture corrosion between sweet and salt water. However, this are only guesses, as far as we know this has not yet been researched.
The waterfilled shaft is used by apnoe divers, which dive as deep as possible along a rope. They do not use any technical aid, no gas tank, only the air in their lungs. As a result they do not need decompression, they simply go down along the rope and then back to the surface, all in a few minutes. You know this probably from the famous Luc Besson movie Le grand bleu. There are six different freediving disciplines: static apnea, dynamic apnea with fins, dynamic apnea without fins (pool disciplines), constant weight apnea with fins, constant weight without fins, and free immersion (depth disciplines).
Dean's Blue Hole is the place where William Trubridge, an apnoe diver from New Zealand, works as an Apnea Academy instructor and operates a freediving school and annual competition, both called Vertical Blue. He holds the world record in the free immersion and the constant weight without fins disciplines, and was the first to break the 100m barrier unassisted. Those records were accomplished at Dean's Blue Hole. However, as his records are in the area around 100m, depending on the discipline, he never reached the ground of the shaft, as this is 200m deep. So it may be sufficient for further world records, which probably explains why this spot is so popular.