Zacatón

Listen first. The air here is still and warm, and it smells faintly of sulphur, like a struck match left to cool. Below the rim, the water lies dark and round, almost perfectly circular. Small mats of grass float on its surface and drift slowly. They move because the wind moves them... there is no current you can see. This is Zacatón, in the Mexican state of Tamaulipas, near the town of Aldama.

At first sight, it looks like a quiet pond in a ring of rock and scrub. That impression is wrong. Zacatón is a cenote, a water-filled sinkhole in limestone country. It is one of the deepest water-filled sinkholes ever measured on Earth. The pool you see covers only a small circle of surface. Beneath it, the water drops straight down into darkness for hundreds of metres.

The name comes from those floating islands. In Mexican Spanish, zacate means grass or pasture grass. Zacatón refers to the tall grass that grows in clumps here. Some of those clumps break free from the edges and become floating rafts. They wander across the pool for months or years. Locals say the islands never stay in one place. From the rim, you can watch them shift position over a single afternoon.

Zacatón does not stand alone. It is part of a group of sinkholes and pools in this area, and scientists often call them the Zacatón system. They lie on private ranch land known as Rancho La Azufrosa. The ranch name comes from azufre, the Spanish word for sulphur. That smell in the air is the same thing. The water here carries dissolved minerals and gases from deep underground.

This is the first surprise most visitors miss. Most famous cenotes, like those in the Yucatán Peninsula, formed when rainwater slowly dissolved limestone. Zacatón seems to have a different origin. Researchers believe warm, acidic water rising from below helped carve it. This region has a volcanic past, and that heat still reaches the groundwater. So the pool is warm. It formed partly from the bottom up, not only from the top down.

Look at the walls of the shaft. The rock is pale and layered, with rough ledges and dark stains near the waterline. Some of this rock is travertine, a kind of limestone that forms when minerals settle out of mineral-rich water. In other words, the same water that dissolved the rock in some places also built new rock in others. The sinkhole is still a working system. It changes slowly, but it does change.

For decades, nobody knew how deep Zacatón really went. Divers tried to find out. In 1994, two of the world's leading deep divers came here, the American Jim Bowden and the American Sheck Exley. Bowden reached a depth of more than 280 metres, a record at the time. Exley did not return from his dive. He was a pioneer of cave diving, and he died in this water in April of that year.

After that dive, Zacatón became a place of serious respect among divers. Even Bowden did not reach the bottom. The shaft simply continued below him. The pressure at those depths is enormous, and the breathing gases needed are complex and dangerous. Human divers could not answer the question safely. So the answer had to come from a machine.

In 2007, a team supported by NASA lowered a robot into the water. It was called DEPTHX, short for Deep Phreatic Thermal Explorer. The robot looked like a large orange shape, a little like a flattened egg. It swam on its own, without a pilot, and used sonar to map the walls around it. It measured the depth of Zacatón at about 335 metres. That is deeper than many tall skyscrapers are high.

Why would a space agency care about a pond in Tamaulipas? Here is the second surprise. NASA scientists wanted to test technology for exploring the oceans of other worlds. Jupiter's moon Europa is thought to have liquid water under a thick shell of ice. A robot that can explore a dark, warm, mineral-rich shaft on Earth is good practice. The team also collected samples of microbes living in the water. Some of those microbes were new to science.

Those microbes form colourful mats on the rocks near the surface. Colours can include purple, green, and orange, depending on light and depth. They live on chemical energy as well as sunlight. Life in this water does not depend on the same conditions as life in an ordinary lake. For biologists, Zacatón is a natural laboratory. Every sample adds to our understanding of where life can survive.

A practical note. This is private land, and access is not guaranteed. Diving is tightly controlled and is not for casual visitors. If you are here, you are probably standing on the rim, as a guest of the ranch or as part of an organised visit. Stay well back from the edge. The rock can be loose, and the drop to the water is long. Respect the rules of the owners. They protect the site for everyone.

The surrounding countryside is dry and open. Low thorn scrub and grassland stretch away under a wide sky. In the heat of the day, insects buzz in the brush and birds call from the trees. The sinkhole sits in this landscape almost without warning. One moment you walk through ordinary ranch country. The next moment, the ground opens into a deep blue-black circle.

Before you leave, look once more at the surface. Find one of the grass islands and follow it for a minute. It moves slowly across water that hides a drop of more than three hundred metres. Feel the warmth in the air and notice the faint smell of sulphur. Then turn back toward the ranch track. The pool will look small behind you... but now you know how far down it goes.