Zacatón

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. The 392 meters deep Pozzo del Merro is deeper, but its nature as a near-vertical cave or a sinkhole still being debated.Using an autonomous robot, the underwater portion of Zacatón has been measured to be 319 meters deep. Zacatón is the only sinkhole of the five located in Rancho La Azufrosa to have any noticeable water flow.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.

Source: Wikipedia