The Great Blue Hole is a large marine sinkhole off the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll 70 km from the mainland and Belize City. The hole is circular in shape, 318 m across and 124 m deep. It has a surface area of 70,650 square metres. It was formed during several phases of the Quaternary glaciation when sea levels were much lower.