Dean's Blue Hole is the world's deepest known "Blue Hole" with an entrance below the sea level. It plunges 663 Feet deep (202 meters) in a bay west of Clarence Town on Long Island, Bahamas.

"Blue Hole" is a term which often is given to sinkholes filled with water, with the entrance below the water level. They can be formed in different karst processes, for example, by the rainwater soaking through fractures of limestone bedrock onto the water table. 

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. The maximum depth of most other known blue holes and sinkholes is 360 feet (110 meters), which makes the 663 feet (202 meters) depth of Dean's Blue Hole quite exceptional.

Dean's Blue Hole is roughly circular at the surface, with a diameter ranging from 82–115 feet (25 to 35 meters). After descending 66 feet (20 meters), the hole widens considerably into a cavern with a diameter of 330 feet (100 meters).

There are several freshwater water-filled sinkholes that are deeper than Dean's Blue Hole. These include the 890 feet (270 meters) Boesmansgat in South Africa, Mexico's Zacatón at 1099 feet (335 meters) and the 1286 feet (392 meters) Pozzo del Merro in Italy.