Jacob’s Well in Texas (photograph by Patrick Lewis)



There are places in the world where it feels like you can swim forever into the Earth. Known as blue holes, the geological phenomenon is when a cave or sinkhole fills with water and becomes a vertical void in the landscape. Some descend hundreds of feet, others connect to mysterious systems of tunnels. And some people are brave enough to swim into the gaping abyss.

Below are six of the world’s most fascinating blue holes:

GREAT BLUE HOLE

Belize

photograph by wstera2/Flickr user

We start with the largest: the Great Blue Hole off the coast of Belize. The underwater sinkhole is 410 feet deep, and 984 feet wide, and looks like a massive blue chasm opening in the sea. It’s ringed by a coral reef, and was formed during the last glacial period when a limestone cavern collapsed into a “vertical cave.” As you descend, the geological formations become more otherworldly and complex.

photograph by Robert Scales

photograph by John C. Bullas

DEAN’S BLUE HOLE

Bahamas

photograph by Christian Afonso

But for the deepest blue hole in the sea, you have to head to the Bahamas in a bay west of Clarence Town on Long Island. Dean’s Blue Hole is a flooded sinkhole 663 feet deep, nearly twice as deep as the blue hole average of 360 feet. As you go down into the void, the cave expands from 82 feet in diameter to 330 feet. It was here that diver William Trubridge set a free-diving record of 331 feet on just one breath, but it was also the site of a tragedy that called into question the practice of such treacherous diving. In November of last year, diver Nicholas Mevoli died after an attempt to break that record.

William Trubridge in Dean’s Blue Hole (photograph by Igor Liberti)

RED SEA BLUE HOLE

Dahab, Egypt

photograph by Mark Edley

While diving in any blue hole has its dangers, the Blue Hole of the Red Sea in Dahab, Egypt, is infamous as the “Diver’s Cemetery.” It’s estimated that at least 40 divers have perished in its depths, although some people think it’s twice that many, and there’s even a memorial ridge overlooking the water with commemorations for the lost. Nevertheless, divers are there each day, attempting to swim some of the 462 feet depth of the blue hole, and the elusive tunnel through a reef that links it to open water.

photograph by mindgrow/Flickr user

photograph by Matt Kieffer

Memorials to dead divers (photograph by Tim Sheerman-Chase)

photograph by Jürgen Donauer

photograph by Tim Sheerman-Chase

photograph by Mark Edley

BLUE HOLE AT THE AZURE WINDOW

Dwerja, Malta

photograph by Darren Barefoot

The Azure Window is a 328-foot archway that towers over the Blue Hole in Gozo, part of the Maltese Islands. The two geological phenomena side-by-side make the site especially popular with divers and tourists, and octopi, fire worms, and even the occasional sea horse can be seen enjoying the waters. To access the blue hole you journey through a 262-foot tunnel that connects two underground limestone caverns.



photograph by Sebastian Sachse



photograph by Michael Aston





photograph by Martin Lopatka



photograph by David Locke

SANTA ROSA BLUE HOLE

Santa Rosa, New Mexico



photograph by davidd/Flickr user

The Santa Rosa Blue Hole might not look like much compared to other blue holes on this list, but the swimming hole located off of Route 66 in New Mexico has a secret. The beautifully clear waters go down to 80 feet, but then the caves begin. It’s unclear exactly how far back the cave system goes, and in 1976 two young divers perished while exploring. A grate was then placed over the entrance and the caves were almost entirely forgotten until the ADM Exploration Foundation investigated last year. Even then much of the tunnels was left unseen, their secrets still hidden for now while up above swimmers and scuba diving students enjoy the blue waters without knowing their true depths.



photograph by davidd/Flickr user



photograph by davidd/Flickr user

photograph by Autopilot/Wikimedia

JACOB’S WELL

Wimberley, Texas



photograph by Patrick Lewis

Another swimming hole — Jacob’s Well in Texas — is popular with scuba divers and swimmers with its 30-foot-drop, but also incredibly dangerous. A dozen people have suffocated after becoming trapped in the narrow caves hidden in the waters, and a local diver even went so far as to install a grate over the entrance to stop further tragedies. However, he later found it removed and a plastic note in the water with these words: ”You can’t keep us out.”

photograph by Patrick Lewis

photograph by Patrick Lewis

Explore even more blue holes on the Atlas Obscura >