I am about to climb down a 10-metre rope ladder into a manhole-sized gap in the floor of the Mexican jungle on the Yucatán peninsula, to dive an underwater cave system, exploring paths where no one has ever been in modern history.

You may be familiar with cenotes, or sink holes, the beautiful wells filled with tempting blue water, but this hole does not look like that. Why would anyone want to head down there is a good question but we are exploring places where no one has been since the Mayans. This place doesn’t exist on any maps. My objective is to explore places with the view to bringing about environmental awareness, hopefully resulting in protection.

I am with fellow cave diver and explorer Alex Reato, in his 4x4 pick-up loaded with equipment, and accompanied by our four porters. As we head into the jungle the vegetation around the road gets denser and denser.

We get out, put on mosquito repellant and start walking into the jungle, carrying camera kit, dive tanks and GPS sets. The GPS point is our only real reference in the jungle as the vegetation is all too similar to navigate by.

Walking in the denser jungle, I notice structural outlines in the floor. Despite now being covered in soil and leaves it is fairly easy to see the rectangular shapes of what must had been the foundations of Mayan houses.

The Mayans were one of the most advanced indigenous cultures of the ancient Americas. But at the end of the eighth century, something unknown happened. One by one, the cities were abandoned and Mayan civilisation collapsed. A prominent theory is that the Maya had exhausted the environment around them to the point that it could no longer sustain a very large population. If anything, we have not learned from history as we are now doing the same thing on a planetary scale.

We attach a rope ladder to the base of a thick tree, put on our wetsuits and equipment and climb down the tight hole. It is deep and narrow, and gets darker and darker. I exit the shaft and enter a cave, the roof is about two metres above the surface of the water, the inside of the cave is about 40m in diameter and, given how small the shaft is, this feels even more impressive. The water we are standing in is the entrance to a vast underwater river system, there are thousands of kilometres of submerged caves on the Yucátan and cave exploration is a little bit like connecting the dots.

Underwater cave exploration on the Yucátan only started in the 1980s with American cave divers who used small single-engine aircrafts to fly over the jungle trying to spot cenotes visible from the air, then throw toilet rolls down for marking the spot. Next they would walk into the jungle to find the cenote they had spotted and marked.

The full extent of the rivers is still not clearly understood. The extensive sub-horizontal flooded cave network is a 8-12km-wide strip of the Caribbean coastline. Combined, these rivers form the aquifer of Yucatán and support about 11 different ecosystems.

After we enter our hole, we try to squeeze through tight caves underwater and find a passage that connects to the other system. The cave is dark, but panning the lights around it looks very impressive, there are small bats flying round, there is an island with stalagmites and stalactites that must have formed during the little ice age when the water level was lower. I can also see roots of trees reaching the water.

Being underground feels isolated but not at all detached. The rock that separates us is karst rock, a very porous limestone allowing water to seep through easily.

The porters now hoist down our four tanks, two for each of us. We attach them to our harness in the water, getting ready to dive.

We start the exploration a little bit away from the cave entrance, Alex attaches a new guideline and starts swimming into the unknown. He is using his compass to try to find a connection to the known mapped caves. On land this would be simple, but in underwater caves it is not easy.

Trying to find a passage in here and exploring it is all about flow: an indicator that water is moving and that the cave is part of the bigger underwater river system. The cave gets even more narrow and there is very little visibility as silt from our movements and bubbles cloud the water. In situations like this, cave diving is more a mental exercise than a physical one.

I continue squeezing through this tight space and after about 10-12m of crawling we swim into an open cave with about 2m of vertical space, which feels luxurious. Some of these bigger spaces can be incredible beautiful, big rooms can open up with otherworldly limestone decorations. We continue exploring trying to find a connection, but the passages to the north are too tight and it is time to turn around as we have used up a third of our air.

Heading back out there is zero visibility. I cannot see my hand, I can see my shoulder but not my elbow. Going in through the long tight passage we stirred up silt and that has not settled yet. Even as we now move incredibly carefully, the air bubble we exhaled set off a rain of dust coming off the cave ceiling so we are almost blind.

Exploring in these conditions requires mental resilience.

If we just wanted to get out we could just follow the guidline, but we have a mission. On the way out measurements are taken.

We follow the line back all the way to the big open underground cave where we entered. There is still plenty of air left in our tanks so we explore the perimeters of the cavern. I want to take another look at the rock pile and get a picture of it. Looking at this from the side it is apparent it must be man-made; the top flat slaps are in a position that would be perfect for angling a wooden ladder to the small hole with access to the jungle.

Alex is signalling with his dive light - he has found a fully intact clay pot. We know we are the only two people to have been here for at least 500 years and finding this artefact makes the whole thing quite humbling.

What is truly fascinating about the underwater rivers is that modern impact has so far not destroyed them the way most other ecosystems around the world are being ruined at the moment. I am trying to make sure they don’t get irreparably polluted or damaged. Exploring them puts them on the map. This in turn theoretically means they have some sort of protection from new development under Mexican law.

Exiting the jungle with a plastic slate containing about 120 numbers written in pencil may seem like a naive way to try to solve such a big issue, but this is how the mapping is done. Back at home, Alex puts the numbers into software to process into a stick map. It can be viewed individually, or together with all the other existing maps. Google Earth is used for placing the maps on top of the landscape, so the arteries of the jungle become visible. Looking at satellite images we also see the hotels and other development.

My hope is that by bringing back stories, pictures and maps, we can help explain what a unique hidden world is right under our feet. Admittedly it can only really be accessed by cave divers, but the implications and importance are massive. There is a widespread impact as 10 million people visit this area every year, and about 2 million locals depend on the aquifer for drinking water. Tourism on the Yucatán is the second biggest contributor to Mexico’s GNP and the Mesoamerican reef is the world’s second largest reef. All the issues are interconnected.

In my view it is not too much to say that the whole Yucatán depends on the aquifer for survival, yet there is immense pressure on the area to the point where it’s not sustainable. The tourism industry brings in millions of tourists every year but many hotels don’t treat the sewage water. Other contaminants include illicit drugs, pharmaceuticals, shampoo, toothpaste, pesticides, chemical run-off from highways and personal care products from domestic sewage. The many golf courses pollute with fertiliser run-off that flows through the systems, spurring algae growth on the reef, further stressing the corals.

The bottom line is that waste contaminates the vast labyrinths of water-filled caves and polluted water flows through them and into the Caribbean Sea. Land-sourced pollution may have contributed (along with other factors), to the loss of 50% of corals on the reefs since 1990. In 1996 it was designated as a world heritage site by Unesco, it is the habitat for more than 500 fish species, five species of marine turtles and attracts one of the world’s largest congregations of whale sharks.

In addition, the submerged caves are essentially a huge archaeological site, partly unexplored. Most importantly we have a chance to preserve one of the few places on the planet we have not yet devastated.



