Beneath the idyllic resort towns of Mexico’s Yucatan Peninsula, daring explorers have uncovered a hidden world of grand chambers and twisting tunnels.

The Ox Bel Ha, Mayan for ‘Three Paths of Water’, is a sprawling water ‘web’ that makes up the world’s longest underwater cave system.

In this vast network, researchers have found giant sink holes, huge crystal chambers known as ice palaces, and 38 unique species of cave–dwelling animals.

Cave divers have mapped out 325.6 miles (524 kilometres) of subterranean tunnels, but believe there could be even more yet to explore.

Since 2018 alone, divers have squeezed their way through 108 more miles (175 kilometres) of unexplored cave.

Research group, El Centro Investigador del Sistema Acuífero de Quintana Roo (CINDAQ), believes it may find more than 373 miles (600 kilometres) of cave before its investigations are done.

The Ox Bel Ha is the second–longest explored cave in the world, and is only beaten by Kentucky’s Mammoth Caves, which run for at least 426 miles (686 km).

The big difference is that every metre of Mexico’s cavernous network is submerged in a mixture of salt and freshwater.

The Ox Bel Ha is the world’s largest cave network, extending at least 325 miles beneath Mexico’s Yucatan Peninsula 

Scientists believe that the incredible complexity of Ox Bel Ha is due to the unusual way in which it formed.

Millions of years ago, rainwater and dissolved carbon dioxide filtered through a thick layer of limestone on the Yucatan Peninsula.

As this acidic solution filtered through the rock, it reacted with the limestone and carved out vast channels and tunnels.

Then, approximately 18,000 years ago, severe climate changes caused ocean levels to rise and flood the caves – sealing the underground landscape exactly as it is found today.

Rather uniquely, this is also one of the few underwater caves in Mexico which connect directly to the sea.

This means that there is a layer of faster–flowing fresh water sitting on top of a nearly static saltwater layer.

This creates what scientists call a ‘halocline’, a layer where salt and fresh water mix, which makes exploration even trickier.

Exploration only began on the Ox Bel Ha cave network in 1996, when explorers found an entrance inside one of the area’s natural sink holes, known as cenotes.

The leading research group, El Centro Investigador del Sistema Acuífero de Quintana Roo (CINDAQ), believes they may find more than 373 miles (600 kilometres) of cave before their investigations are done

Exploration is continuously uncovering new chambers and tunnels as explorers find more connections between different sections of the cave

The only cave system longer than Ox Bel Ha is Mammoth Cave in Kentucky (pictured), which extends for at least 426 miles (686 km)

Exploring a dry cave network, such as Mammoth Cave, is already dangerous and difficult.

Cavers must force themselves into narrow passages known as constrictions in the hope that they will open up enough to turn around further on.

Becoming trapped or lost can easily result in the caver’s death if they are not able to call for help.

Underwater, these difficulties are all massively increased by the fact that explorers must also manage their limited oxygen supply.

Cave divers must balance the amount of gas they bring with them against the fact that they may need to pull that equipment through some tight squeezes.

This makes long dives extending deep underground extremely challenging, and something that can only be attempted by the most skilled divers.

However, over the last 30 years, explorers have gradually managed to piece together a map of the Ox Bel Ha.

As the divers explore, they trail a line of cable behind them that allows them to find their way back, and also to assess how long the cave network is.

Ox Bel Ha is home to huge sink holes, 38 species of unique cave–dwelling animals, and a series of crystal–coated chambers known as an ‘ice palace’ (pictured) 

Since 2018 alone, divers have squeezed their way through 108 more miles (175 kilometres) of unexplored cave

And, as divers have repeatedly resurveyed old sections of cable, they are still finding entirely new sections of tunnel to explore.

For example, in 2022, a group of divers were following an old cable about two miles deep into the cave when they discovered that it had been overlooked by earlier dives.

The divers told In Depth Magazine at the time: ‘To our surprise it looked like we were the first ones there.’

By undertaking mammoth six–to eight–hour dives, the explorers pushed up to 6,370 metres into the cave and uncovered 6.2 miles (10.1 kilometres) of new passageways.

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However, the efforts to map the extent of Ox Bel Ha have not been without fatalities.

In April 2022, an experienced Russian diver who had been mapping the cave for six months set out on a dive from sinkhole Cenote Odyssey and never returned.

Investigators found the diver floating face down within arm’s reach of a spare gas tank, with his own tank drained empty.

It is believed that the diver became trapped in a cloud of silt while exploring a new crack, panicked, and used up the remainder of their air supplies faster than they had been prepared for.