A Collapsed Ice Age Cavern
The Great Blue Hole is a massive marine sinkhole, or cenote, located 70 kilometers (43 miles) off the coast of Belize. This near-perfect circle, 318 meters (1,043 feet) across and 124 meters (407 feet) deep, records the planet's glacial past. It is part of the Belize Barrier Reef Reserve System, a UNESCO World Heritage Site.
Its story began during the Quaternary glaciation, a period of repeated ice ages. Analysis of its submerged stalactites shows the original limestone cave system formed in several stages, with major events occurring 153,000, 66,000, 60,000, and 15,000 years ago. During these periods, global sea levels were much lower, and this structure was a dry cave on land. Acidic rainwater slowly dissolved the limestone bedrock, carving out huge chambers and forming the dripstone features common to caves.
As the last Ice Age ended and glaciers melted, the rising Caribbean Sea flooded the cave system. The roof, weakened by its vast size, could not support the weight of the water above and collapsed, creating the vertical, water-filled cavern seen today. Explorer Jacques Cousteau brought his research vessel, the Calypso, here in 1971, confirming its origin as a drowned karst cave and declaring it one of the world's top diving sites.
Life and Death in the Abyss
The upper rim of the Great Blue Hole is alive with corals and is a habitat for numerous fish and shark species, including Caribbean reef sharks, nurse sharks, bull sharks, and hammerheads. But this bright ecosystem exists only in the sunlit upper layer.
Descending into the hole shows a change. At a depth of approximately 90 meters (300 feet), the water is stratified with a thick layer of hydrogen sulfide. This toxic layer prevents oxygen from circulating to the bottom, creating an anoxic, or oxygen-free, environment below. Life ceases to exist in this dark, still water.
A 2018 expedition using submersibles created the first complete 3D sonar map of the hole's interior. The team discovered that the bottom is not flat but covered in sediment and the debris from the ancient roof collapse. They also found a "conch graveyard," where hundreds of mollusks had fallen into the hole, become trapped below the hydrogen sulfide layer, and suffocated. Tiny tracks in the sediment show where the conchs futilely attempted to crawl back up the steep inclines. The expedition also made the discovery of plastic bottles and the remains of two divers who had gone missing in the hole.