Zacatón (El Zacatón sinkhole) is a thermal water filled sinkhole belonging to the Zacatón system - a group of unusual karst features located in Aldama Municipality in the northeastern state of Tamaulipas, Mexico. It is the deepest known water-filled sinkhole in the world with a total depth of 339 metres (1,112 ft).[1] The 392 metres (1,286 ft) deep Pozzo Merro (Italy) is deeper but is rather a near-vertical cave and not a sinkhole.
Using an autonomous robot, the underwater portion of Zacatón has been measured to be 319 metres (1,047 ft) deep (a 20-metre (66 ft) difference between the rim of cliff and surface of water adds to the total depth).[2] Zacatón is the only sinkhole of the five located in Rancho La Azufrosa to have any noticeable water flow.[3]
The name Zacatón comes from the free-floating islands of zacate grass which move around on the surface with the wind.[4]
Scrapings from the rock walls beneath the surface yielded at least 6 new phyla of bacteria.
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El Zacatón's depth has made it an important dive site:
Zacatón is just one of the numerous sinkholes and other karst features in the region. Here are located more than 15 sinkholes, several cave systems and karst springs with caves.[14] Several of these karst features have unusual characteristics like travertine lids over several of the sinkholes with isolated waterbodies below.
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.[15] 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.[16] 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.[3] 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.[17]
