A blue hole is a cave (inland) or underwater sinkhole. They are also called vertical caves. There are many different blue holes located around the World, typically in low-lying coastal regions. The best known examples can be found in Belize, the Bahamas, Guam, Australia (in the Great Barrier Reef), and Egypt (in the Red Sea).
The deepest blue hole in the worldat 202 metres (663 ft)is Dean's Blue Hole, located in a bay west of Clarence Town on Long Island, Bahamas. Other blue holes are about half that depth at around 100120 metres (330390 ft). The diameter of the top entrance ranges typically from 2535 metres (82115 ft) (Dean's Blue Hole) to 300 metres (980 ft) (Great Blue Hole in Belize).