A blue hole is a large marine cavern or sinkhole, which is open to the surface and has developed in a bank or island composed of a carbonate bedrock (limestone or coral reef). Their existence was first discovered in the late 20th century by fishermen and recreational divers. Blue holes typically contain tidally influenced water of fresh, marine, or mixed chemistry. They extend below sea level for most of their depth and may provide access to submerged cave passages. Well-known examples are the Dragon Hole (in the South China Sea) and, in the Caribbean, the Great Blue Hole and Dean's Blue Hole.
Blue holes are distinguished from cenotes in that the latter are inland voids usually containing fresh groundwater rather than seawater.Their water circulation is poor, and they are commonly anoxic below a certain depth; this environment is unfavorable for most sea life, but nonetheless can support large numbers of bacteria. The deep blue color is caused by the high transparency of water and bright white carbonate sand.
The diameter of the top entrance ranges typically from 25–35 metres (82–115 ft) (Dean's Blue Hole) to 300 metres (980 ft) (Great Blue Hole in Belize). The overall largest blue hole (taking into account depth and width) is located 100 kilometers from the coast of Belize. The Great Blue Hole is a massive 300 meters wide and 125 meters deep.
In 2018, another group of scientists set out to explore the Great Blue Hole of Belize using two submarines of the latest technology. One of the major scientific contributions to result from this expedition was the first 3-dimensional map of its interior. The researchers captured features such as stalactites, the hydrogen sulfide layer, and other details that cannot usually be seen by the naked human eye.
Source : Blue Hole
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