Blue hole

A deep marine sinkhole appearing as a dark blue circle in shallow water; the Great Blue Hole in Belize, 124 m deep, is the most famous dive.

A blue hole is a vertical marine sinkhole or collapsed cave shaft, visible from above as a startling circle of deep blue set in pale shallow water. They formed during ice ages, when sea level was far lower and rainwater dissolved shafts in exposed limestone; when the ocean rose again, the caverns flooded. The Great Blue Hole in Belize, about 300 m across and 124 m deep, is the icon, alongside Dean's Blue Hole in the Bahamas at 202 m and Egypt's Blue Hole at Dahab.

The classic Great Blue Hole dive drops to around 40 m to see stalactites hanging under a cavern ledge, proof the shaft was once dry cave. These dives are short, deep, and dark, run tightly on the no-decompression limit, and often plagued by nitrogen narcosis rather than marine life, since blue hole water is frequently still and low in oxygen at depth.

Blue holes deserve respect: Dahab's Blue Hole has claimed many divers who attempted the deep arch beyond recreational limits. Dived within your certification and plan, they are safe, atmospheric, bucket-list sites, and freedivers treasure them too, Dean's Blue Hole hosts world-championship competitions. Book with operators who enforce depth limits, run thorough briefings, and check certifications, the good ones all do.