Great Blue Hole
Sinkholes are typically found on land, but off the coast of Belize one can be found in the middle of the Caribbean Sea. When this unusual hole was formed, it was actually located above sea level; at the time, much of the Earth's water was contained in glaciers and ocean levels were low. During one of these dry periods, a 1000-foot (305-meter) wide section of topsoil fell 412 feet (126 meters) into a cave below, creating the large, cylindrical shaft [source: Belize Audubon Society]. Today the sinkhole -- the largest of its kind -- lies under shallow water and is a part of the Lighthouse Reef, located some 55 miles (89 kilometers) east of Belize City. These two features are included in the Belize Barrier Reef Reserve System, a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Site.
The Great Blue Hole, whose deep blue color stands in sharp contrast to the light blue lagoon that surrounds it, has become a very popular scuba diving destination. French adventurer Jacques Cousteau introduced the location to the world in 1971 on his television show, "The Undersea World of Jacques-Yves Cousteau." Since then, thousands of divers have come to explore the hole's impressive diversity of marine life as well as geological features from its days above water, including stalactites, dripstone sheets, and columns. In 2009, UNESCO placed the Belize Barrier Reef Reserve System, which includes the Great Blue Hole, on its List of World Heritage in Danger, due to mangrove cutting and excessive development in the area.