Drowned at the end of the Ice Age: The flooded abyss longer than the Grand Canyon and teeming with hundreds of creatures, from mammals to reptiles

Picture making your way through a vast cave system. The darkness has an eerie greenish glow and, in places, light streams down like a beam from a spaceship. You pass stalagmites standing tall and stalactites gripping tightly as they dangle from the cave roof. 

But you’re not walking through the network of tight passages that link huge caverns. You’re swimming – or, more precisely, scuba diving. This might give you a glimpse of what it’s like to visit Sistema Ox Bel Ha near Tulum, Mexico. 

This complex tangle of flooded passages is currently the world’s longest explored underwater cave system. Its name means ‘Three Paths of Water’ in the ancient Mayan language. 

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It’s thought that these caves have been underwater for 18,000 years since changes in sea level caused them to flood.

Cave divers only started exploring this tangle of subterranean tunnels in 1998 – and for good reason. “From where it is most easily accessible, a side road, the cave is marked by a group of small, complex and unattractive tunnels,” writes Global Underwater Explorers on its website.

“This alone was enough to turn away the less determined cave diver, especially since there were a number of highly decorated cave systems under active exploration at the time,” they add. “In comparison to these other systems, Ox Bel Ha seemed to be a bleak prospect.”

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But when cave divers entered, they discovered a unique and extensive ecosystem.

Some channels are directly linked to the Caribbean Sea which brings saltwater into the caves. But there is also freshwater that comes from rain. These form two distinct layers: the heavier saltwater sinking below the freshwater. The point at which they meet – called the halocline – has a shimmering quality.

Many animals call this strange place home. According to Global Underwater Explorers, the area has 550 species of birds, 103 types of mammals, 182 reptiles and amphibians, 36 crustaceans and two fish, along with various plants. 

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Visiting is not for the faint-hearted. “This extensive cave system offers complex navigational challenges and stunning cave formations, but it is suitable only for experienced cave divers,” writes SSI on its website. 

But, for some, the potential danger is worth the risk. “Its immense size, mix of tight passages, and large chambers adorned with impressive stalactites make this an exceptional dive site,” adds SSI.

But Ox Bel Ha has a contender to its crown of world’s longest underwater cave: Sistema Sac Actun. Historically, this was believed to be two separate cave systems: Sistema Sac Actun (White Cave System) and the Dos Ojos (Two Eyes). But in 2018 scuba divers discovered a channel that connected the two and the combined length made it the world’s longest underwater cave… temporarily, at least.

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“As of July 2019, the combined total length of this underwater cave system has now been measured to stretch 371.958 kilometres (231.12 miles), as confirmed by the local Quintana Roo Speleological Survey,” writes Guinness World Records on its webpage for the Longest underwater cave system explored. The page names Sistema Ox Bel Ha as the previous record holder with around 270 kilometres (168 miles) of passages.

Then, another plot twist – Ox Bel Ha took the lead again. 

As of 15^th August 2026, Cave Exploring’s list of the world’s longest caves ranks Sistema Ox Bel Ha as the longest underwater system at 336.65 miles long. Sistema Sac Actun takes second place at 234.07 miles. For context, the Grand Canyon is 277 miles long.

The only cave system currently known to be longer than Ox Bel Ha is Mammoth Cave in Kentucky, USA. This system is a staggering 426 miles long but, being on land, it slightly easier to explore than its underwater counterparts. 

With records frequently changing as new passages are discovered connecting cave systems once thought to be distinct, which will end up on top? Only time will tell. 

top image: Mexico, Yucatan, Tulum, cave diver in the system Tux Kubaxa. Not part of the Sistema Ox Bel Ha. Credit: Westend61/ Getty Images

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