The Great Blue Hole, located near Ambergris Caye, Belize.
A blue hole is a submarine cave or sinkhole. They are also called vertical caves. There are many different Blue Holes located around the world, such as in the Bahamas and in Belize.
Blue holes are roughly circular, steep-walled depressions, and so named for the dramatic contrast between the dark blue, deep waters of their depths and the lighter blue of the shallows around them. 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 deepest blue hole in the world - at 202 metres (663 ft) - is Dean's Blue Hole, located in a bay west of Clarence Town on Long Island, Bahamas. The next deepest holes are about half that deep at around 100–120 metres (330–390 ft).