The Great Blue Hole is located in Belize Barrier Reef Reserve System, about 60 miles away from Belize City. It is believed that this hole is the world’s largest sea-hole. It is about 125 meters deep and its diameter is about 300 meters wide. It has been created as a cause of sea level increase about 65,000 years ago.
Deep sea divers are able to explore some of them.some blue holes, however, contain ancient fossil remains that have been discovered, preserved in their depths.
The deepest blue hole in the world—at 202 metres is Dean’s Blue Hole, located in a bay west of Clarence Town on Long Island, Bahamas
Courtesy : wikipedia