Natural Landscapes: The Great Blue Hole, Belize

This underwater sinkhole is 1,000 feet wide and 375 feet deep, and a major scuba diving attraction. It is part of the Belize Barrier Reef, which is in turn part of the Mesoamerican Reef. This hole is believed to have formed during the recent ice ages, when a submerged limestone cave system collapsed due to changes in the sea level. Huge stalactites and stalagmites are found in the hole, which contain records of past climates.

The Great Blue Hole is a giant submarine sinkhole off the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll 43 miles from the mainland and Belize City. It was formed during several episodes of quaternary glaciation when sea levels were much lower. 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. 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 (UNESCO).

This site was made famous by Jacques 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 69 ft, 161 ft, and 299 ft. 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, 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 410 ft 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 407 ft at its deepest point. This difference in measurement can be explained by ongoing sedimentation or by imprecision in measurements. The expedition's goal was to collect core samples from the Blue Hole's floor and document the cave system. To accomplish these tasks, all of the divers had to be certified in cave diving and mixed gases.

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.

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".

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.

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