The Maya Called These Sinkholes “Gateways to the Underworld” — Now Divers Swim Through Them

Imagine standing at the edge of a pool of water so clear it doesn’t look real. Not clear like a swimming pool — clear like the water isn’t there. You can see every ripple of light, every fish, every stalactite reaching down from the ceiling, down to a depth of thirty metres. Then you look closer, and you realize the pool isn’t a pool at all. It’s a window into a flooded cave system stretching for miles beneath your feet.

Welcome to a cenote. The Yucatán Peninsula in Mexico is riddled with thousands of them, and they are among the most beautiful — and strangest — places on the planet. If you love the idea of underground worlds that feel alien, New Zealand’s glowing Waitomo caves are the other great example — but cenotes are the ones you can swim through.

What a cenote actually is

The word “cenote” comes from the Mayan dzonot, meaning “sacred well.” Geologically, a cenote is what happens when limestone bedrock collapses over an underground water-filled cave. The Yucatán’s bedrock is basically one giant slab of limestone, porous as a sponge, with no rivers on the surface — all the water flows underground. Over millennia, acidic water dissolved the rock, carving caverns — a process not unlike how underwater caves are formed around the world. When a cavern’s roof fell in, the groundwater rose up to meet the sky. Cenote.

There are four main types, and they’re all worth knowing because they feel like completely different worlds:

• Open cenotes, like natural amphitheatres — big open pools surrounded by jungle.
• Semi-open cenotes, where the collapse is partial and tree roots dangle into the water.
• Cavern cenotes, entered through a small opening into an underground chamber.
• Ancient cenotes, the fully enclosed ones, where you swim or dive through rock passages into cathedral-like rooms.

Some are shallow swimming holes. Others are entrances to cave systems so vast that explorers have mapped hundreds of kilometres of flooded passages and still haven’t found the end.

The Maya saw them as doors to the underworld

Here’s where cenotes stop being a geology lesson and become something darker. To the ancient Maya, who lived with almost no surface water, cenotes were life itself — the only reliable freshwater. But they were also holy. And terrifying.

In Mayan cosmology, the underworld was called Xibalba — “the place of fright” — and cenotes were its doorways. The Maya made offerings to the rain god Chaac at cenotes: jade, gold, pottery, incense. And, at times, people. The Sacred Cenote at Chichén Itzá — a gaping pit nearly 60 metres across — was the centre of it all. For over a century, archaeologists dismissed the human sacrifice stories as Spanish propaganda. Then divers started pulling human remains from the bottom, some of them children, along with ceremonial artefacts. The bones didn’t lie.

Standing at the edge of that cenote today, in the UNESCO-listed ruins of Chichén Itzá, you feel it. The water is black-green, perfectly still, surrounded by a sheer rock wall. It’s beautiful and deeply wrong at the same time. The Maya weren’t stupid for fearing it. They were right.

Swimming through a sunbeam

I should be honest about the other side of this, though. Most cenotes you’ll visit aren’t sacrificial pits. They’re paradise.

Swim into a cavern cenote on a sunny morning and you’ll see something that doesn’t happen anywhere else: light shafts. Because the water is filtered through limestone for decades before it reaches the pool, it’s almost perfectly transparent. Sunlight drops through the opening above in solid-looking beams, and suspended particles drift through them like dust in a cathedral. You can freedive down through a sunbeam and it feels — and I know how this sounds — like flying.

Then there’s the halocline. In coastal cenotes, a layer of fresh water floats on top of intruding seawater. Dive through the boundary and your vision warps: everything shimmers and blurs like a heat mirage, because the two waters bend light differently. It’s one of the strangest sensations in diving. You feel like you’ve crossed into another medium entirely.

The formations, too. When sea levels were lower during the last ice age, these caves were dry, and stalactites and stalagmites grew inside them. Then the seas rose, the caves flooded, and the formations were drowned intact. You can dive past stalactites that grew when mammoths walked the surface. The caves are time capsules with the water still in them.

The divers mapping the underworld

Not everyone is content to swim in the pretty parts. The Yucatán’s flooded caves have drawn a small, obsessive community of cave divers who’ve been mapping the systems for decades. Two of them — Sac Actun and Ox Bel Ha — have each been surveyed to well over 300 kilometres of connected passages, making them among the longest underwater cave systems in the world. In 2018, explorers proved that Sac Actun and Dos Ojos connect, creating a single system of at least 347 kilometres.

Mapping a flooded cave is slow, cold, dangerous work. Divers lay guidelines as they go, spooling line through passages where a single wrong turn in zero visibility could be fatal. They work in teams, with redundant gas supplies, stage bottles cached along the route. The survey data they bring back has rewritten the map of the Yucatán’s underground — and it’s still growing. Nobody believes the full extent is known.

And here’s a detail I love: inside some of these passages, divers have found the bones of Ice Age animals — giant ground sloths, ancient elephants, even human remains over 12,000 years old, like the famous “Naia” skeleton found in the Hoyo Negro pit. The caves flooded after these creatures fell in. They’ve been waiting down there ever since.

How to visit a cenote without being that tourist

Cenotes are popular, which means some of the famous ones get crowded, and the fragile water can suffer. Here’s how to do it right:

• Pick the less-famous ones. Everyone goes to Ik Kil (the famous one near Chichén Itzá). Fewer people visit the quieter cenotes around Tulum, Coba, or Valladolid — and some are just as beautiful.
• Shower before you swim. Most cenotes ask you to rinse off first. Sunscreen and bug spray contaminate the water. This isn’t a suggestion; the ecosystem depends on it.
• Don’t touch the formations. They took thousands of years to grow and seconds to break. This goes double if you’re diving.
• Hire a local guide for cavern diving. Cenote diving that goes beyond the daylight zone requires cave-diving certification. Don’t improvise. The world’s most dangerous caves have killed confident divers who underestimated them — flooded passages give no second chances.
• Go early. Cenotes open around 9 or 10 am. Arrive at opening and you’ll often have the place almost to yourself. The light is better anyway — those sunbeams are strongest mid-morning.

A doorway, still open

The Maya believed cenotes connected the world of the living to the world of the dead. Standing in the cool, clear water, looking up at jungle framed in a circle of sky, with the dark passages of a flooded cave waiting below — I’m not sure they were entirely wrong. A cenote is a threshold. You cross from heat to cool, from noise to silence, from the surface world to something older.

Most tourists will float, take their photos, and leave. But if you linger, if you dive down just a little and look into the dark mouth of the cave, you’ll feel it: the sense that the Earth goes deeper than you thought, and that people have been standing exactly where you are, feeling exactly what you feel, for thousands of years.

The door to the underworld is still open. It charges about ten dollars admission. Go.