The Mysterious Belize Great Blue Hole is a large underwater hole off the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll 100 kilometres (62 mi) from the mainland and Belize City. The hole is perfectly circular in shape, over 300 metres (1000 ft) across and 125 metres (410 ft) deep.
It was formed as a limestone cave system during the last glacial period when the sea level was 400 to 500 feet below present time and was dryland. Last glacial period began about 120,000 years ago and end about 15,000 years ago. Reaching the maximum extension 26,500 years ago. At the end the ocean began to rise, the caves flooded, and the roof collapsed. Believed to be the world's largest feature of its kind, the Great Blue Hole is part of the larger Belize Barrier Reef Reserve System, a World Heritage site of the United Nations Educational, Scientific and Cultural Organization (UNESCO).

The hole itself is the opening to a system of caves and passageway that penetrate this undersea mountain. In various places, massive limestone stalactites hang down from what was once the ceiling of air-filled caves thousand of years before the end of the last Ice Age 15,000 years ago. When the ice melted the sea level rose, flooding the caves. This process occurred in stages. Evidence for this are the shelves and ledges, carved into the limestone by the sea, which run the complete interior circumference of the Blue Hole at various depths. The Blue Hole is a "karst- eroded sinkhole." It was once a cave at the center of an underground tunnel complex whose ceiling collapsed. Some of the tunnels are thought to be linked right through to the mainland, though this has never been conclusively proved. The mainland itself has many water-filled sinkholes that are connected to caves and tunnels. Notable are the large population of sharks such as lemon, black tip, reef, hammerhead, and bull sharks.

Mysterious and legends always have been around the Belize Blue Hole. It's the kind of underwater geology that inspires speculation about aliens creating geometrically perfect anomalies and monsters living in darkness.

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