Plunging to a depth of 663 feet (202 meters) in a bay west of Clarence Town on Long Island, Dean’s Blue Hole is one of the deepest known blue holes. A blue hole is a term that is often given to sinkholes that are filled with water, while the entrance to them is located beneath the surface. 

While most other known blue holes only reach maximum depths of 360 feet, the 663 foot depth of Dean’s Blue Hole makes it quite exceptional. At the surface, Dean’s Blue Hole is roughly circular with a diameter ranging from 82-115 feet (25-35 meters). After one descends 66 feet below the surface, the hole widens considerably into a cavern with a diameter of 330 feet (100 meters). The water at Dean’s Blue Hole is extremely clear and on good days the visibility can reach 115 feet.

In April 2010, professional diver William Trubridge broke a free-diving world record in the blue hole by reaching a depth of 302 feet (92 meters) without the use of fins. He took that even further when on December 14, 2010, he swam to a depth of 331 feet (101 meters) on a single breath while using only his hands and feet for propulsion. In November of 2013, however, free-diver Nicholas Mevoli died during an effort to dive to a new American record.