There’s no rule that says the great wonders of the natural world have
to be on dry land. Described by legendary underwater explorer
Jacques Cousteau as one of the ten best diving sites on the planet, the Great Blue Hole, situated off the coast of Belize in Central America, is believed to be the world’s largest submarine sinkhole. Some 300 metres wide and 125 metres deep, the UNESCO World Heritage Site is a near-perfect circular formation where the seafloor drops sharply away from the surrounding shallows of the Lighthouse Reef atoll.
The warm waters are a beautiful, clear blue. But it wasn’t just the scenery that got Cousteau excited when he took his ship, the Calypso, there in 1971. The Great Blue Hole is also a paradise for geologists, who can marvel at its unusual rock formations and intriguing history. This hole in the ground was formed in drier times. In past ice ages, the sea level was 100 to 120 metres lower than it is now, and so the area around the Great Blue Hole would have been dry land. Cave systems formed in the limestone, and when the ceiling of one cave collapsed as a result of water erosion, a sinkhole formed. It was submerged when the ice melted and the sea rose to its current level.
The presence of stalactites provides evidence of this formation’s past, as they cannot form underwater. Some are five degrees from vertical and show a consistent orientation, indicating that the underlying rock has tilted at some point. Fossils found in blue holes can also provide clues about their past. At Sawmill Sink on Great Abaco Island in the Bahamas, skeletons of a tortoise and a crocodile – along with the bones of a lizard, snakes, bats and over 25 species of birds – were found by University of Florida researchers. A human tibia was also recovered. The fossils are the best preserved of any ever found in the Bahamas thanks to their location in a deep layer of the sinkhole with little oxygen. Oxygen would normally feed the bacteria and fungi that cause bones to decay.