The Great Blue Hole, which is situated off the coast of Belize, is a massive submarine sinkhole lying near the middle part of the Lighthouse Reef. The hole is round, and spans 300 meters or 984 feet wide in diameter and goes as far as 124 meters or 407 feet deep.
This sinkhole was formed after a number of episodes of quaternary glaciation. During this time, the sea levels were significantly lower. The formation happened several times starting from 153,000 years ago. When the ocean begins to rise, the cave is flooded. This place has been declared as a World Heritage Site by the UNESCO.