Extreme Diving By Guillaume Néry

Dean's Blue Hole is the world's deepest known blue hole, or underwater sinkhole. It plunges 202 metres (663 ft) in a bay west of Clarence Town on Long Island, Bahamas. Blue holes are the results of rainwater having soaked through fractures of limestone bedrock onto the watertable of glacial sea levels during the Pleistocene epoch (ice age), some 15,000 years ago. The maximum depth of most other known blue holes and sinkholes is 110 metres (360 ft), which makes the 202 metres (663 ft) depth of Dean's Blue Hole quite exceptional.
Dean's Blue Hole is roughly circular at the surface, with a diameter ranging from 25 to 35 metres (82–115 ft). After descending 20 metres (66 ft), the hole widens considerably into a cavern with a diameter of 100 metres (330 ft). Sinkholes are created in a similar fashion, although their entrance is generally above sea level. One notable sinkhole is Zacatón in Mexico -- even deeper than Dean's Blue hole -- with a vertical drop of 335 metres (1,099 ft).