The Great Blue Hole is a large submarine sinkhole off the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll 70 km from the mainland and Belize City. The hole is circular in shape, over 300 m across and 124 m deep. It was formed during several episodes of quaternary glaciation when sea levels were much lower. Analysis of stalactites found in Great Blue Hole shows that formation took place 153,000; 66,000; 60,000; and 15,000 years ago. As the ocean began to rise again, the cave was flooded. The Great Blue Hole is a part of the larger Belize Barrier Reef Reserve System, a World Heritage Site of the United Nations Educational, Scientific and Cultural Organization.
This site was made famous by Jacques-Yves Cousteau, who declared it one of the top ten scuba diving sites in the world. In 1971, he brought his ship, the Calypso, to the hole to chart its depths. 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, 49 m, and 91 m . Stalactites were retrieved from submerged caves, confirming their previous formation above sea level. Some of these stalactites were also off-vertical by 5˚ in a consistent orientation, thus indicating that there had also been some past geological shift and tilting of the underlying plateau, followed by a long period in the current plane.
Initial measured depth of Great Blue Hole was 125 m which is the most often cited depth up to this day. An expedition by the Cambrian Foundation in 1997 measured the hole's depth as 124 m at its deepest point. This difference in measurement can be explained by ongoing sedimentation or by imprecision in measurements. The purpose of this expedition was the collection of core samples from the floor of the Blue Hole and documentation of the cave system. To accomplish these tasks, all of the divers had to be certified in cave diving and mixed gases.
The actual name of “The Great Blue Hole” was created by British diver and author Ned Middleton after having lived in the country for 6 months. So impressed with this natural feature, he reasoned in his book “Ten Years Underwater” that if Australia could have ‘The Great Barrier Reef’ then Belize could equally have ‘The Great Blue Hole’ – thus setting this feature apart from similar, albeit lesser in size, structures. This was later reinforced by his second book “Diving Belize”.
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 juvenile fish species . Other species of sharks, such as the bull shark and hammerheads, have been reported there, but are not regularly sighted. Usually, day trips to the Great Blue Hole are full-day trips from the coastal tourist communities in Belize. The trips normally include: one dive in the Blue Hole, a dive on Half Moon Wall and Dive sites to the East of Long Caye.
Diving has also been offered from the island Long Caye in the Lighthouse Reef only a few kilometers away from the Great Blue Hole. The dive operator on Long Caye offers two dives in the Blue Hole: North side; and South side.
On-shore caves of similar formation, as large collapsed sinkholes, are well known in Belize and in the Yucatán Peninsula, where they are known as cenotes. Unlike the mainland cenotes which often link to underwater cave systems, there is little evidence of horizontal development in the Blue Hole.
In 2012, Discovery Channel ranked the Great Blue Hole as number one on its list of "The 10 Most Amazing Places on Earth".
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