<P> Dean's Blue Hole is the world's second deepest known salt water blue hole with an entrance below the sea level . It plunges 202 metres (663 ft) in a bay west of Clarence Town on Long Island, Bahamas . </P> <P> A blue hole is a water - filled sinkhole with the entrance below the water level . They can be formed in different karst processes, for example, by rainwater soaking through fractures of limestone bedrock onto the watertable . Sea level here has changed: for example, during the glacial age during the Pleistocene epoch (ice age), some 15,000 years ago, sea level was considerably lower . </P>

Dean’s blue hole is one of deepest