What are blue holes?
The most abundant examples of these striking geological phenomena are found in and around the islands of the Bahamas. Some 300,000 years ago, when the ice age caused the ice caps to grow and the sea level to fall by up to 120 metres (394 feet), conditions in the Bahamas were well-suited to the formation of underwater caves known as blue holes. A type of karst formation, the blue hole forms as a result of recently exposed soluble rock – such as the limestone uncovered due to a drop in sea level – being eroded by acidic groundwater and rain, which enters through open faults. This causes cavities, caverns and networks of underground tunnels that weaken the structure of the limestone until it collapses in on itself as a sinkhole. When the sea level rises again the entrance to the cave below the surface of the ocean becomes apparent due to the contrast of the deep dark blue of the sinkhole with the lighter shallows of the tropical oceans surrounding it.
One of the most remarkable examples of a blue hole on Earth is the practically perfect circle of the Great Blue Hole off the coast of Belize; at over 300 metres (1,000 feet) across it’s also the world’s largest. Underwater sinkholes can be as deep as several hundred metres and, due to their hostile conditions, pose an extremely dangerous challenge to even the most experienced divers.