The area's location over the Huasteca Karst and the erosion of limestone has given rise to numerous pit caves. The largest of these in the Sierra Gorda is located in Arroyo Seco, called the Sótano de Barro, located in the Santa María de los Cocos community. The Sótano del Barro is one of the largest cavities of the world by volume, measuring 15 million cubic metres, surpassed only the Sima Mayor de Sarisariñama in Venezuela. In comparison, the Sótano de Las Golondrinas has a volume of only five million meters cubed. Its widest diameter measures 420 meters and its average width is 200 meters. It has a total depth of 455 meters with a direct drop of 410 meters. Its perimeter is surrounded by dense vegetation, which is a sanctuary for green macaws. It requires a walk or mule ride of about two hours through dense forest to reach. The walls of the pit cave are mostly vertical and partially covered by vegetation. At the bottom of the cave there is a small forest on a floor which was sea bed 100 million years ago, which measures 220 meters long and 100 meters wide. Despite its depth, the bottom of the pit cave receives an abundance of sunlight allowing for the growth of trees and other vegetation, making it an isolated mini forest. The bottom also has a number of small caves, with the largest fifty meters long. This pit cave has been classified as the deepest in the world although this has recently been challenged by those who wish to include those pit caves which are not continuous such as the Stary Swistak in Austria, which has a total depth of 480 meters.