The Great Blue Hole is a giant submarine sinkhole that is situated off the coast of Belize. The Great Blue Hole lays in the centre of the Lighthouse reef that is 70 km from the mainland of Belize. The circular hole is 300 metres in diameter and almost 108 metres deep. The hole was said to have formed during the quaternary glaciation, when sea levels were lower. Originally a cave, the Great Blue Hole was formed when the cave began to flood with ocean water.