Hierve el Agua (Spanish for "the water boils") is a set of natural rock formations in the Mexican state of Oaxaca that resemble cascades of water. The site is located about 70 km east of Oaxaca city, past Mitla, in the municipality of San Lorenzo Albarradas, with a narrow, winding unpaved road leading to the site. The site consists of two rock shelves or cliffs which rise between fifty and ninety metres from the valley below, from which extend nearly white rock formations which look like waterfalls. These formations are created by fresh water springs, whose water is over-saturated with calcium carbonate and other minerals. As the water scurries over the cliffs, the excess minerals are deposited, much in the same manner that stalactites and iron can also be found and determines the color of a particular stalactite. The calcium carbonate in the water is due to rainwater which passes underground. First it absorbs carbon dioxide, and forms molecules of carbonic acid. This acid comes into contact with underground marble and partially dissolves creating calcium bicarbonate. When the water runs above ground, the excess minerals fall out.
The waters, with their high mineral content, are reputed to have healing qualities.
The more easily accessible and more often visited of the two waterfalls is the "cascada chica," also called the Amphitheatre. This cliff rises over fifty metres above the valley floor, where there is a natural platform about sixty meters wide. This platform has four springs. The water from three of the four springs is captured by a number of small natural pools and two large artificial pools in which visitors can swim. The lower of the two pools is very close to the cliff edge and was built in 2007. In these pools, the water appears a turquoise green due to the mineral concentration. The fourth spring is located closest to the edge and responsible for most of the waterfall rock formation. Two of the four springs on the Amphitheatre side rise from the level floor and appear to bubble up, which is the source of the name. The bubbling action of these and some other springs is thought to occur due to construction of underground water currents. This bubbling action leads to the name of the area, Hierve el Agua (the water boils). Water constantly flows out of the springs and the process of depositing the calcium carbonate is ongoing, which can be seen on the walls of the artificial pools.
Trails lead from the cascada chica to the cascada grande, the valley floor and other areas in the vicinity. The cascada grande is just to the south of the cascada chica and easily visible from it. This waterfall rock formation is more vertical than the cascada chica. Similarly, it is a rock shelf from which flows mineral laden water over the side. This shelf is ninety metres above the valley floor, eighty metres wide with the waterfall extending down for about thirty metres. This shelf does not have artificial pools.