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Did you know that the Bahamas have the greatest concentration of blue holes in the world? 

A blue hole is a cave or underwater sinkhole. They are also called vertical caves. They are named for the dramatic contrast between the dark blue, deep waters of their depths and the lighter blue of the shallows around them. The deep blue color is caused by the high transparency of water and bright white carbonate sand. Pictured above is the deepest blue hole in the world with underwater entrance — at 202 metres (663 ft) — called Dean’s Blue Hole, located in a bay west of Clarence Town on Long Island, Bahamas. (Read more on BCC and NatGeo)