The Great Blue Hole is a giant submarine sinkhole off the coast of Belize. This site was made famous by Jacques Cousteau, who declared it one of the top ten scuba diving sites in the world. 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 108 m (354 ft) deep. It was formed during several episodes of quaternary glaciation when sea levels were much lower. As the ocean began to rise again, the cave was flooded. The Great Blue Hole is a part of the larger Belize Barrier Reef Reserve System, a World Heritage Site of the United Nations Educational, Scientific and Cultural Organization (UNESCO).