The Great Blue Hole is a large underwater sinkhole off of the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll 45 miles from the mainland and Belize City. The hole is circular in shape, over 1,000 feet across and 400 feet deep. It was formed as a limestone cave system during the last ice age when sea levels were much lower. As the ocean began to rise again the caves flooded, and the roof collapsed.