What is a blue hole?

A blue hole is a deep marine sinkhole — a vertical underwater cavern that opens suddenly in shallow seabed. From above it reads as a circle of dark navy in turquoise water, which is exactly how it got the name. Blue holes formed during the ice ages, when sea levels sat far lower and rain carved caves into exposed limestone; when the oceans rose, the caves flooded. The Bahamas hold the world’s densest concentration, including Dean’s Blue Hole on Long Island, which plunges about 663 feet. Belize’s Great Blue Hole is the most photographed. Snorkelers hang at the rim over the drop. Freedivers go down it. Explore the Exuma Cays by yacht.