El Zacatón, a vertical cave about 100 meters (328 feet) wide and more than 300 meters (1,000 feet) deep, has been called an “upside down Mount Everest.” It could easily swallow New York’s Chrysler Building. No one has ever reached the bottom, so it’s true depth is unknown.

The caves and sinkholes of the Zacatón system are located on a ranch near the small town of Aldama in the state of Tamaulipas in northeastern Mexico.

these caves and sinkholes formed as a result of volcanic activity from below. This is very different from the classical theory of how caves form and from how most of the other large caves in this part of Mexico likely formed

The sinkholes also display another interesting characteristic. They appear to be in varying stages of closing up as crusts of travertine form at their surfaces. At least one appears to have already closed up entirely, sealing off an underwater lake, possibly with unusual life forms.