The Great Blue Hole is a large submarine sinkhole off the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll 70 km (43 mi) from the mainland and Belize City. The hole is circular in shape, over 300 m (984 ft) across and 124 m (407 ft) deep. It was formed during several episodes of quaternary glaciation when sea levels were much lower. Read more at http://en.wikipedia.org/wiki/Great_Blue_Hole