El Zacatón is a thermal water-filled sinkhole belonging to the Zacatón system – a group of unusual karst features located in Aldama Municipality near the Sierra de Tamaulipas in the northeastern state of Tamaulipas, Mexico. It is the deepest known water-filled sinkhole in the world with a total depth of 339 meters.
Using an autonomous robot, the underwater portion of Zacatón has been measured to be 319 meters deep (a 20-meter difference between the rim of the cliff and the surface of the water adds to the total depth). Zacatón is the only sinkhole of the five located in Rancho La Azufrosa to have any noticeable water flow.
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. Several of these karst features have unusual characteristics like travertine lids over several of the sinkholes with isolated waterbodies below.
The name Zacatón comes from the free-floating islands of zacate grass which move around on the surface with the wind. Scrapings from the rock walls beneath the surface yielded at least three new phyla of bacteria.
According to Wikipedia