The Great Blue Hole is a giant marine sinkhole off the coast of Belize.
It is a part of the larger Belize Barrier Reef Reserve System, a World Heritage Site of the United Nations Educational, Scientific and Cultural Organization (UNESCO).
The Belize Barrier Reef is a series of coral reefs straddling the coast of Belize, roughly 300 meters (980 feet) offshore in the north and 40 kilometers (25 miles) in the south within the country limits. The Belize Barrier Reef is a 300-kilometer (190 miles) long section of the 900-kilometer (560 miles) Mesoamerican Barrier Reef System; the largest barrier reef in the Western Hemisphere and the second largest in the world, after Australia’s Great Barrier Reef.
The Great Blue Hole lies near the center of Lighthouse Reef, a small atoll 70 kilometers (43 miles) from the mainland and Belize City.
The hole is circular in shape, 318 metes (1,043 feet) across and 124 meters (407 feet) deep.
It is the world’s largest natural formation of its kind.
First explorations of this unique structure were made by legendary Jaques-Yves Cousteau in 1971. He and his team from ship Calypso charted the depth of the hole in 1971. They found that this is karst formation which formed in at least four stages.
Investigations by this expedition confirmed the hole’s origin as typical karst limestone formations, formed before rises in sea level in at least four stages, leaving ledges at depths of 21 m (69 ft), 49 m (161 ft), and 91 m (299 ft).
Analysis of stalactites found in the Great Blue Hole shows that formation took place 153,000; 66,000; 60,000; and 15,000 years ago.
The circulation of water in these sinkholes is very poor, since their shape prevents much current flow. After a certain depth the waters are often anoxic, meaning that they are depleted of oxygen.
This is a popular spot among recreational scuba divers who are lured by the opportunity to dive in sometimes crystal-clear water and meet several species of fish, including Midnight Parrotfish, Caribbean reef shark, and other fish species.
Even though the Great Blue Hole is considered a bucket-list dive, one should be aware that it is not for divers of all skill levels – a prerequisite is logging more than 24 dives. It is also not a ‘colorful dive’, instead, divers witness a dark cave with impressive stalactites.
In 2012 Discovery Channel ranked the Great Blue Hole as number one on its list of “The 10 Most Amazing Places on Earth“.