The Great Blue Hole is a large submarine sinkhole

Blue hole

A blue hole is a cave or underwater sinkhole. They are also called vertical caves. There are many different blue holes located around the world, typically in low-lying coastal regions...

 off the coast of Belize

Belize

Belize is a constitutional monarchy and the northernmost country in Central America. Belize has a diverse society, comprising many cultures and languages. Even though Kriol and Spanish are spoken among the population, Belize is the only country in Central America where English is the official...

. It lies near the center of Lighthouse Reef, a small atoll

Atoll

An atoll is a coral island that encircles a lagoon partially or completely.- Usage :The word atoll comes from the Dhivehi word atholhu OED...

 70 kilometres (43 mi) from the mainland and Belize City

Belize City

Belize City is the largest city in the Central American nation of Belize. Unofficial estimates place the population of Belize City at 70,000 or more. It is located at the mouth of the Belize River on the coast of the Caribbean. The city is the country's principal port and its financial and...

. The hole is circular in shape, over 300 metres (984 ft) across and 124 metres (407 ft) deep. It was formed during several episodes of Quaternary glaciation

Quaternary glaciation

Quaternary glaciation, also known as the Pleistocene glaciation, the current ice age or simply the ice age, refers to the period of the last few million years in which permanent ice sheets were established in Antarctica and perhaps Greenland, and fluctuating ice sheets have occurred elsewhere...

 when sea level

Sea level

Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

s were much lower - the analysis of stalactites found in Great Blue Hole shows that formation has taken place 153,000, 66,000, 60,000, and 15,000 years ago. As the ocean began to rise again, the caves were flooded. The Great Blue Hole is a part of the larger Belize Barrier Reef Reserve System

Belize Barrier Reef

The Belize Barrier Reef is a series of coral reefs straddling the coast of Belize, roughly offshore in the north and in the south within the country limits...

, a World Heritage Site

World Heritage Site

A UNESCO World Heritage Site is a place that is listed by the UNESCO as of special cultural or physical significance...

 of the United Nations Educational, Scientific and Cultural Organization (UNESCO

UNESCO

The United Nations Educational, Scientific and Cultural Organization is a specialized agency of the United Nations...

).

Exploration

This site was made famous by Jacques-Yves Cousteau

Jacques-Yves Cousteau

Jacques-Yves Cousteau was a French naval officer, explorer, ecologist, filmmaker, innovator, scientist, photographer, author and researcher who studied the sea and all forms of life in water...

, who declared it one of the top ten scuba diving

Scuba diving

Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....

 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

Karst topography

Karst topography is a geologic formation shaped by the dissolution of a layer or layers of soluble bedrock, usually carbonate rock such as limestone or dolomite, but has also been documented for weathering resistant rocks like quartzite given the right conditions.Due to subterranean drainage, there...

 limestone formations, formed before rises in sea level in at least four stages, leaving ledges at depths of 21, 49, and 91 meters (69, 161 and 299 ft). Stalactite

Stalactite

A stalactite , "to drip", and meaning "that which drips") is a type of speleothem that hangs from the ceiling of limestone caves. It is a type of dripstone...

s 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 metres (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 124 metres (407 ft) at its deepest point. This difference in measurement can be explained by ongoing sedimentation or by imprecision in earlier measurements. The purpose of this expedition was the collection of core sample

Core sample

A core sample is a cylindrical section of a naturally occurring substance. Most core samples are obtained by drilling with special drills into the substance, for example sediment or rock, with a hollow steel tube called a core drill. The hole made for the core sample is called the "core hole". A...

s from the floor of the Blue Hole and documentation of the cave

Cave

A cave or cavern is a natural underground space large enough for a human to enter. The term applies to natural cavities some part of which is in total darkness. The word cave also includes smaller spaces like rock shelters, sea caves, and grottos.Speleology is the science of exploration and study...

 system. To accomplish these tasks, all of the divers had to be certified in cave diving

Cave diving

Cave diving is a type of technical diving in which specialized equipment is used to enable the exploration of caves which are at least partially filled with water. In the United Kingdom it is an extension of the more common sport of caving, and in the United States an extension of the more common...

 and mixed gases.

Tourism

This is a popular spot amongst recreational scuba divers, who are lured by the opportunity to dive in crystal-clear water and meet several species of fish, including giant groupers, nurse shark

Nurse shark

The nurse shark, Ginglymostoma cirratum, sometimes referred to as the Nur Shark is a shark in the nurse sharks family, the only member of its genus Ginglymostoma...

s and several types of reef sharks such as the Caribbean reef shark

Caribbean reef shark

The Caribbean reef shark is a species of requiem shark, family Carcharhinidae. It is found in the tropical waters of the western Atlantic Ocean from Florida to Brazil, and is the most commonly encountered reef shark in the Caribbean Sea...

 and the Blacktip shark

Blacktip shark

The blacktip shark is a species of requiem shark, family Carcharhinidae. It is common to coastal tropical and subtropical waters around the world, including brackish habitats. Genetic analyses have revealed substantial variation within this species, with populations from the western Atlantic Ocean...

. Other species of sharks, like the bull shark

Bull shark

The bull shark, Carcharhinus leucas, also known as Zambezi shark or unofficially known as Zambi in Africa and Nicaragua shark in Nicaragua, is a shark common worldwide in warm, shallow waters along coasts and in rivers...

 and hammerheads

Hammerhead shark

The hammerhead sharks are a group of sharks in the family Sphyrnidae, so named for the unusual and distinctive structure of their heads, which are flattened and laterally extended into a "hammer" shape called a "cephalofoil". Most hammerhead species are placed in the genus Sphyrna while the...

, have been reported there, but are not regular sightings. Usually, dive trips to the Great Blue Hole are full-day trips, which include one dive in the Blue Hole and two further dives in nearby reefs.
Recently diving has also been offered from the island Long Caye in the Lighthouse Reef only a couple of miles away from the Great Blue Hole. They offer 2 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

Yucatán Peninsula

The Yucatán Peninsula, in southeastern Mexico, separates the Caribbean Sea from the Gulf of Mexico, with the northern coastline on the Yucatán Channel...

, where they are known as cenote

Cenote

A cenote is a deep natural pit, or sinkhole, characteristic of Mexico and Central America, resulting from the collapse of limestone bedrock that exposes groundwater underneath...

s.