Zacatón

Zacatón is one of a group of five interconnected sinkholes, or cenotes, located in the northeastern state of Tamaulipas, Mexico. It is the deepest water-filled sinkhole in the world with a total depth of 335 metres (1,099 ft). DEPTHX, a NASA funded project using an autonomous robot has measured the underwater portion to be 319 metres (1,047 ft) deep (an air-filled 16-metre (52 ft) drop from the surface to the water accounts for the total depth).

The name Zacatón comes from the free-floating islands of zacate grass which move about on the surface with the wind.

Scrapings from the rock walls beneath the surface yielded at least 6 new phyla of bacteria.

Diving

El Zacatón's depth has made it an important dive site:

• Dr. Ann Kristovich set the women's world depth record of 554 feet (169 m) during a 1993 dive into the sinkhole.

• On April 6, 1994, explorer diver Jim Bowden and cave diving pioneer Sheck Exley plunged into El Zacatón with the intent of reaching bottom. Bowden dove to a men's world record depth of 925 feet (282 m), but Exley died, probably from high pressure nervous syndrome (HPNS) at a depth of 879 to 906 feet (268 to 276 m).

• The NASA Deep Phreatic Thermal Explorer (DEPTHX) project used the sinkhole as a testbed for robotic hardware being developed to explore the Jovian moon of Europa. DEPTHX was designed and integrated by NASA Principal Investigator Stone Aerospace. Additional Co-Is on the Deep Phreatic Thermal Explorer (DEPTHX) team included Carnegie Mellon University, Southwest Research Institute, Colorado School of Mines, The University of Arizona, and the University of Texas at Austin's Jackson School of Geosciences.

Hydrogeology

Since the late 1990s, Marcus Gary, a hydrogeology graduate student at the Jackson School of Geosciences at the University of Texas at Austin has studied Sistema Zacatón to better understand how the sinkholes formed and how they evolve over time. During these studies, Gary made extensive use of high-end water quality monitoring instrumentation provided free of charge by (the now defunct) Hydrolab Corporation. Gary was a primary member on the DEPTHX mission, which used an autonomous underwater robot to explore the deepest parts of Zacatón for the first time.

According to Gary, these sinkholes began to form during the Pleistocene as a result of volcanic activity from below. This view differs from the classical theory of cave formation and also hypotheses about the formation of other large caves in this part of Mexico.

In this view, volcanism turned deep water slightly acidic by adding dissolved carbon dioxide and hydrogen sulfide. This water slowly dissolved the limestone above, creating porous karst. This is referred to as “hypogenic karstification.” From time to time, overlying rock collapsed into hollow chambers below, creating deep shafts. If his interpretation is correct, Sistema Zacatón has more in common with Mammoth Hot Springs in Yellowstone than with other deep sinkholes in this same region of Mexico.

Another interesting hydrogeologic feature is that some of the sinkholes appear to be in the process of closing up at the top as crusts of travertine (a form of calcium carbonate) form at their surfaces. Marc Airhart, a science writer at the Jackson School of Geosciences wrote:

It’s a bit like the skin on a can of paint that has been left open in the sun. For the paint, it might take a day or two. In this case, the process probably takes thousands of years. It is basically the sinkhole’s way of taking a bunch of dissolved rock floating in the water and recycling it to form new rock at the surface. At least one sinkhole (Poza Seca) appears to have closed up entirely, sealing off an underwater lake, possibly with unusual life forms. If such life forms exist, they’re likely to be bacteria that can live without oxygen and sunlight. And assuming the lake has been sealed off from the outside world for thousands and thousands of years, they might have evolved to be different from anything scientists have ever discovered and characterized before.

Further reading

• Sistema Zacatón
• Dispatches from Zacatón JSG Online, March/May 2007
• NASA-funded Robotic Sub Makes Final Dive To Reach Bottom of Earth’s Deepest Sinkhole JSG Online, May 14, 2007