The Great Blue Hole

The short answer

The Great Blue Hole is a large circular marine sinkhole in the lagoon of Lighthouse Reef, an atoll about 80 kilometres east of the Belizean mainland. The management plan for the site gives it as about 320 metres across at the rim and an estimated 124 metres deep. It formed during several phases of the Quaternary glaciations, when sea levels were much lower and the limestone was a dry cave; as the ocean rose again, the cave flooded. It is a Natural Monument within the Belize Barrier Reef Reserve System, which UNESCO listed as a World Heritage Site in 1996, and it was made famous among divers by Jacques Cousteau, who took his ship, the Calypso, there to chart it in 1971.

Why it's worth knowing

Most people know it as a photograph: a navy-blue circle in a pale reef, taken from a small plane. The photograph is of something that used to be dry.

Here is the evidence. Investigations by Cousteau’s expedition confirmed the hole’s origin as typical karst limestone, formed before the sea rose in at least four stages, which left ledges in the wall at depths of 21, 49 and 91 metres. Stalactites were retrieved from the submerged caves. A stalactite grows by mineral-laden water dripping in air, so finding them underwater confirms that the caves were once above sea level. Some of the older stalactites lean, all in the same direction, which indicates that the rock itself tilted at some point and not only that the sea rose. Wikipedia’s account of the expedition gives the lean as about 5°; the 2017–2021 management plan for the Half Moon Caye and Blue Hole Natural Monuments gives a southward tilt of 12°, and says more recent stalactites grow straight down.

And its records have moved. Its fame rests on size. Wikipedia’s article on blue holes gives its entrance, about 300 metres, as the top of the range for blue-hole entrances, and counts it the largest blue hole when width and depth are taken together. It is not the deepest: in the Taam Ja’ Blue Hole in Chetumal Bay, off the same Caribbean coast, instrument profiles published in 2024 went below 420 metres without reaching the bottom, and the Dragon Hole in the South China Sea is 300.89 metres. The Great Blue Hole is wide; it is not the deepest.

It is also a serious dive. Below about 91 metres the water is dark and anoxic, beneath a layer of hydrogen sulfide.

The field notes

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A cave the sea moved into

Blue holes are roughly circular, steep-walled depressions in carbonate rock. They formed during past ice ages, when sea level was 100 to 120 metres lower than it is now and the limestone was exposed to rain and chemical weathering like any other limestone country. When the sea rose at the end of the ice age the process stopped. They get their name from the contrast between the deep blue of the water in the hole and the lighter blue of the shallows around it; the colour comes from the high transparency of the water and the white carbonate sand.

The Great Blue Hole lies in the atoll’s shallow lagoon. Cousteau’s first sounding put its depth at about 125 metres; the management plan gives an estimated 124 metres, citing a 2000 study by Jones and colleagues, and rounds it to 125 elsewhere.

What is below ninety metres

In December 2018 two submarines descended into the hole to map its interior with sonar. The team came close to a complete three-dimensional map and reported a layer of hydrogen sulfide at about 91 metres. Water at that depth and below is dark, anoxic and devoid of life. Blue holes in general have poor water circulation, and the anoxic water can support large numbers of bacteria that live on sulfur compounds.

An earlier expedition in the summer of 1997 collected core samples from the floor and documented the cave system; all the divers had to be certified in cave diving and mixed gases.

The atoll around it

Lighthouse Reef is the easternmost part of the Belize Barrier Reef and one of its three atolls, with Turneffe Atoll and Glover’s Reef. The management plan gives it as about 35 kilometres long and up to 7.5 wide, about 200 square kilometres in all, around a shallow lagoon of 112. Half Moon Caye, eight miles south of the hole, is another Natural Monument (3,954 hectares, against 414 for the Great Blue Hole). The caye was first protected in 1928 for its red-footed booby colony, which the Belize Audubon Society put at about 3,700 birds in 2015, beside about 1,730 magnificent frigatebirds.

The rim of the hole itself carries a community of coral and tropical fish. The shallow reef edge is about 5 metres deep, slopes to about 10, and then drops away as a near-vertical cliff. Groupers, angelfish and butterflyfish live there, along with nurse sharks, Caribbean reef sharks and blacktip sharks. The atoll has only partial environmental protection, and the article records human impact from uncontrolled fishing and tourism.

Who made it famous

Cousteau brought the Calypso to the hole in 1971 to chart it, and is reported to have declared it one of the top five scuba-diving sites in the world. (Wikipedia’s article on the barrier reef dates his making it famous to 1970; the article on the hole says 1971.) In 2012 the Discovery Channel ranked it first on a list of “The 10 Most Amazing Places on Earth”, and in 2018 it broadcast Discovery Live: Into the Blue Hole, featuring Erika Bergman, Fabien Cousteau and Richard Branson.

A relic from the Great Blue Hole

Getting there

How to get there. The hole is on Lighthouse Reef, out in the Caribbean Sea.

Ways to see it. A flight gives the shape and the colour that the photographs show. A boat takes you to the rim. Snorkelling on the rim and at Half Moon Caye shows you the reef and the fish. Diving the hole means descending along the wall into deep water; ask the operator what certification they require, and about depth limits, gas, briefing and emergency plans, before you book.

When to go. Lighthouse Reef is a marine destination, so sea conditions matter more than the calendar. Ask your operator which months and days give a calm crossing and clear water.

What to book ahead. The flight or boat, and a certified operator. Fees and permits apply in protected areas; ask your operator what is included.

Practical notes. The United States has Belize at Level 2, exercise increased caution, for crime. The one Level 3 area is Southside Belize City, from the Haulover Creek Canal south to Fabers Road, which is not on the route to the hole but is worth knowing about if you connect through the city. The advisory also notes that emergency medical services are usually unavailable or significantly delayed and that serious injuries normally require medical evacuation to another country, which matters for a deep dive.

Common questions

How deep and how wide is the Great Blue Hole?

The Great Blue Hole is about 320 metres across at its rim and an estimated 124 metres deep, according to the 2017–2021 management plan for the Half Moon Caye and Blue Hole Natural Monuments, which elsewhere rounds the figures to 300 and 125 metres. Jacques Cousteau’s 1971 expedition measured about 125 metres, which is still the depth most often quoted. It is a marine sinkhole near the centre of Lighthouse Reef, off the coast of Belize.

How did the Great Blue Hole form?

It began as a limestone cave above sea level during the ice ages, when the sea was 100 to 120 metres lower than today. Investigations by Cousteau’s 1971 expedition confirmed a karst origin, in at least four stages that left ledges at 21, 49 and 91 metres. As the ocean rose after the ice ages, the cave flooded.

Are there stalactites in the Great Blue Hole?

Yes. Stalactites were retrieved from the submerged caves during Cousteau’s 1971 expedition. Because a stalactite grows only in air, they confirm that the caves were above sea level before the ocean rose. Some of the older stalactites lean in one direction, which indicates that the rock itself tilted: Wikipedia’s account of the Cousteau expedition gives about 5°, and the monuments’ management plan a southward tilt of 12°.

What is at the bottom of the Great Blue Hole?

Dark, anoxic water. In December 2018 two submarines mapped the interior with sonar and reported a layer of hydrogen sulfide at about 91 metres, below which the water is devoid of life.

Is the Great Blue Hole the deepest blue hole in the world?

No. In the Taam Ja’ Blue Hole in Chetumal Bay, Mexico, instrument profiles published in 2024 went below 420 metres without reaching the bottom; the Dragon Hole in the South China Sea is 300.89 metres, and Dean’s Blue Hole in the Bahamas is 202 metres. The Great Blue Hole is about 124 metres deep; its entrance, roughly 300 metres across, is at the top of the range Wikipedia gives for blue holes.

Where is the Great Blue Hole?

Near the centre of Lighthouse Reef, the easternmost atoll of the Belize Barrier Reef, in the Caribbean Sea off Belize. Lighthouse Reef is about 80 kilometres east of the mainland coast. Trips leave from Belize City and the northern cayes.

Who made the Great Blue Hole famous?

Jacques Cousteau, who took his ship Calypso to the hole in 1971 to chart its depths and declared it one of the top five scuba-diving sites in the world. Wikipedia’s reef article gives 1970 for his making it famous. In 2012 the Discovery Channel ranked it first on its list of the ten most amazing places on Earth.

Is the Great Blue Hole protected?

Yes. It is a Natural Monument of 414 hectares (1,023 acres), designated in 1996 and co-managed by the Belize Audubon Society, and part of the Belize Barrier Reef Reserve System, which UNESCO designated a World Heritage Site in 1996. Half Moon Caye on the same atoll is a Natural Monument of 3,954 hectares. Wikipedia notes that the atoll has only partial protection and suffers from uncontrolled fishing and tourism.