Great Blue Hole

This is part of the Lighthouse Reef System, The Great Blue Hole is located about 60 miles outside the mainland of Belize City. A large almost perfectly circular hole approximately one quarter of a mile (0.4 km) in, it's one of the most astounding dive sites to be found anywhere on earth.
In this hole, the water is 480 feet (145 m) deep and it is the depth of water that gives a dark blue color that causes such structures throughout the world known as the "blue hole."