El Zacatón

El Zacatón is a thermal water-filled sinkhole on La Azufrosa ranch in the municipality of Aldama, at the foot of the Sierra de Tamaulipas. From the rim it looks like a round crater with rock walls and a dark green pool at the bottom, across which islands of zacate grass drift as the wind pushes them; they give the place its name. What you cannot see is the depth. It is about 20 metres from the cliff edge down to the water, and below the surface an autonomous robot measured another 319 metres, for a total of 339. That puts it among the deepest flooded sinkholes known anywhere.

It belongs to the Zacatón System, a group of five connected cenotes at La Azufrosa (El Zacatón, La Poza Verde, El Caracol, La Pilita and La Azufrosa), and it is the only one of them with noticeable water flow. The area has more than fifteen sinkholes, caves and karst springs, some capped by travertine crusts that are slowly sealing them from the top; one, Poza Seca, has closed completely over an isolated underground lake. Hydrogeologist Marcus Gary, who has studied the system since the late 1990s, argues that these shafts formed differently from other caves in the region: in the Pleistocene, volcanic activity deep below charged the groundwater with carbon dioxide and hydrogen sulphide, which dissolved the limestone from beneath until the roof collapsed into vertical shafts.

El Zacatón has a place in deep-diving history. In 1993 Ann Kristovich set the women's depth record here at 169 metres, and on 6 April 1994 Jim Bowden reached 282 metres, then the men's record, on the same dive in which cave-diving pioneer Sheck Exley died at around 270 metres, probably from high-pressure nervous syndrome. Later NASA's DEPTHX project, built by Stone Aerospace with several US universities, used the sinkhole to test a robot designed to explore Jupiter's moon Europa; its samples turned up about a hundred microbes, including at least three entirely new groups of bacteria.

Today diving is limited to scientific work and highly experienced divers because of the depth and the sulphur in the water. Visitors come to look down from the rim, walk the surroundings and combine it with the other cenotes: La Poza Verde, about 150 metres across and 45 deep, is good for swimming, snorkelling and kayaking, and La Pilita has warm springs. The Los Cuarteles caves are nearby.