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). 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 can be found in Belize, the Bahamas, Guam, Australia (in the Great Barrier Reef), and Egypt (in the Red Sea).
Blue holes are distinguished from cenotes in that the latter are inland voids usually containing fresh groundwater rather than seawater.(Wikipedia)
"The Blue Hole"