A huge marine sinkhole off the coast of Belize and in the Caribbean has awed tourists and scientists for decades. A recent expedition helped uncover its deep secrets.

Exploring Belize's Blue Hole

The Blue Hole is the world's biggest known marine sinkhole, approximately 984 feet in width and 410 feet in depth. It is located offshore in the heart of the Lighthouse Reef, about 40 miles off the coast of Belize, Central America.

That giant underwater hole was made famous by underwater explorer Jacques Cousteau, who declared it one of the top five scuba diving sites in the world. Cousteau sailed his ship Calypso to the hole in 1971 to chart its depths.

Investigations by this mission confirmed the origin of the hole as typical karst limestone formation. The Blue Hole has also become part of the reserve system for the Belize Barrier Reef and a UNESCO World Heritage Site.

In the summer of 1997, the hole's floor was visited again to collect core samples and document its cave system. Yet, so much of this underwater sinkhole was unexplored deep down.

READ ALSO: World's Deepest Sinkhole: How Huge Are These Topographic Depressions?


Mysteries of the Deep

In December 2018, Cousteau's grandson Fabien returned to a dive in the Blue Hole, accompanied by British entrepreneur and fellow explorer Richard Branson, and joined a team of explorers that also included oceanographer Erika Bergman.

The team aimed to capture high-quality photos and videos of the Blue Hole's characteristics with the help of a three-person Aquatica Submarine (Stingray 500). They also used a Roatan Institute of Deepsea Exploration Submarine (Idabel) and sonar technology provided by Kongsberg Mesotoech.

The mission had other plans of getting a 3D map of the inside of the hole. Upon reaching the bottom the startling findings were different instead.

As they edged down the Blue Hole, the first thing they encountered was a wall of giant stalactites. Another challenge they faced at about 300 feet was a thick layer of toxic hydrogen sulfide, which plunged them into an instant darkness and cut out the oxygen from the water around them.

This layer formed over centuries naturally. According to Bergman, all the sunlight will lose and everything will just turn completely black once this layer is pierced.

Furthermore, he stated that the visibility of the surrounding environment was "extremely eerie," and they never expected to see any creatures from below. However, when they finally made it to the bottom, they saw an entire food chain in action: crabs, conches, falling in the hole, reptilian monsters reaching the bottom, running out of oxygen, and dying.

Further down in the abyss, Branson and his fellows kept seeing some remains of a tragedy. Bergman said in an interview in 2020 that they had come across the bodies of two people lost in the Blue Hole.

But at the bottom, they found something else quite unexpected and unwelcome: human rubbish. There was a 2-liter Coke bottle and a lost GoPro containing some holiday photos.

When asked about the mythical monsters of the deep, Branson noted that the real monster threatening the ocean is climate change. Within it holds a complex system of caves that once formed on dry land, thus providing evidence of how oceans can rise quickly and catastrophically. The Blue Hole is, in relation, a proof of how oceans can rise quickly and catastrophically.

Ancient sea levels have been several hundred feet lower than they are now. As much ice melted around the planet ten thousand years ago, the level of the sea increased by about 300 feet. The change in the rock where it used to be land and turned into sea can be seen at 300 feet by explorers.

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