The Great Blue Hole is a large underwater sinkhole off the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll 70 kilometres (43 miles) from the mainland and Belize City. The hole is circular in shape, over 300 metres (984 ft) across and 124 metres (407 ft) deep. It has formed during several episodes of Quaternary glaciation when sea levels were much lower – the analysis of stalactites found in the Great Blue Hole shows that formation has taken place 153,000; 66,000; 60,000; and 15,000 years ago. As the ocean began to rise again, the caves were flooded. The Great Blue Hole is a part of the larger Belize Barrier Reef Reserve System, a UNESCO site.
A blue hole is a submarine cave or underwater sinkhole. They are also called vertical caves. Blue holes are roughly circular, steep-walled depressions, and so named for the dramatic contrast between the dark blue, deep waters of their depths and the lighter blue of the shallows around them. Their water circulation is poor, and they are commonly anoxic below a certain depth; this environment is unfavorable for most sea life, but nonetheless can support large numbers of bacteria. The deep blue colour is caused by the high transparency of water and bright white carbonate sand. Blue light is the most enduring part of the spectrum; where other parts of the spectrum – red, yellow, and finally green – are absorbed during their path through water, blue light manages to reach the white sand and return back upon reflection.