Located near the centre of Lighthouse Reef, a small atoll 100 km (62 mi) from the mainland of Belize City, the Great Blue Hole is a large underwater sinkhole, which is considered as the largest of its kind in the world. The hole, considered as the largest of its kind, is a part of the larger Barrier Reef Reserve System. It is almost circular in shape, over 300 metres (984 ft.) across and 125 metres (410 ft.) deep and is characterized by its rich, blue colour. However, the sinkhole, when viewed from above, has a dark blue tinge because of its depth and it can be easily distinguished in contrast to the shallow aqua blue water of its surroundings.
The Blue hole, a lagoon with light turquoise-coloured water is encircled by a coral island. It is believed that, the sinkhole was originally formed as a limestone cave during the last glacial period, a time when the sea levels were much lower. After the end of the ice age, as glaciers started to melt, the sea level began to rise and ultimately the cave system was flooded. In due course, the roof of the cave became so thin that it collapsed, creating a vast sink hole or a vertical cave in the ocean.
In 1971, Jacques Cousteau, an undersea explorer, made the site famous, when he announced it as one of the top ten best scuba diving places in the world. In fact, he sailed on his ship Calypso, investigated the sinkhole’s depths and confirmed that it had, indeed, originated from a limestone cave formation and formed before the rise in the sea level in at least four stages. Huge stalactites retrieved from submerged caves confirmed their previous formation above the sea level. The tilted position of some of the stalactites indicates that there had also been some past geological shift and tilting of the underlying plateau, followed by a long period in the current plane. The tilt also indicates that this was a movement of the land, rather than a rise in the sea level alone.
The Great Blue Hole is part of the larger Belize Barrier Reef Reserve System, which is a World Heritage Site of the United Nations Educational, Scientific and Cultural Organization (UNESCO). In 2012, it was ranked as number one by the Discovery Channel, among the 10 Most Amazing Places on Earth.