You should visit Hierve el Agua (daily 9am–6pm), the site of a spectacular calcified formation that resembles a bubbling waterfall. The site is some 25km east of Mitla, down a side road that leads to San Lorenzo Albarradas (Hierve el Agua lies just beyond the village). The mineral concentration causes the water to bubble out of the ground and become petrified over the vertiginous cliff-tops, forming a stunning stalactite; it is a beautiful sight and the panoramas from above the pools, where there several stalls serving tacos and other snacks, are jaw-dropping.

Tourism here threatens to be more ecofriendly than in the rest of the valley: San Lorenzo can support the volume of visitors it receives between local villagers over how to split the proceeds check with the state tourism office to see if it’s open. It is advisable to hire a car to get here; the only public transport is provided by camioneta (pick-up truck) from Mitla, which leave when full. There are a few basic cabañas ecoturísticas here (though check in advance to see if they are operating), as well as a few simple comedores, which close after sunset. Alternatively, tour groups from Oaxaca often take in Hierve El Agua as part of day trip that includes Mitla and Teotitlán del Valle.

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 are formed in caves. One of the cliffs, called the "cascada chica" (small waterfall) or the Amphitheatre, contains two large artificial pools for swimming as well as a number of small natural pools. One of the artificial pools is very near the edge of the cliff.

The area is also of archeological interest because of the extensive system of irrigation and terraces built by the Zapotecs as many as 2,500 years ago. It was also most likely a sacred site to the ancient peoples of the Oaxaca valley. The canals are part of an irrigation system which was created more than 2,500 years ago, most of which extend from the Amphitheatre side. Researchers have studied the terraces and canals which have been cut into the sides of the mountains these cliffs are on and have concluded that there were an irrigation system, unique in Mexico. Only vestiges of these irrigation canals remain; however, the archeological area has not been completely explored. These irrigation canals are considered to be unique in Mesoamerica, not only because irrigation was not common in pre-Hispanic Mexico but because they are the only example of lined irrigation canals. It is not known why canal lining does not appear outside of the Hierve el Agua area, and here the lining develops naturally. While seepage and bank erosion were indeed problems with irrigation farming in other parts of Mesoamerica, the nearest example of anything similar is with the Hohokam of southern Arizona in 650 C.E.

The rock formations are often described as "cascadas de sal" (salt waterfalls), "cascadas petrificadas" (petrified waterfalls) and "cascadas pétreas" (rock waterfalls). The falls are formed by relatively small amounts of water which is oversaturated with calcium carbonate and comes to the surface through cracks or figures on the mountainside. The water has a temperature of 22 to 27C. As the water runs down the rock face, it forms large stalactites similar to those found in caves. The flow of water from the spring varies significantly between the dry and rainy seasons. 95% of the surface rock formations are of calcium carbonate; however, lower layers of both rock and water currents are little known as they are sealed by the above layers. The calcium carbonate gives the formations a white or near white appearance, but other minerals that can be present in the water, such as silver, barium 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 site consist of two waterfall-like rock formations which have been formed over thousands of years. Both cliffs rise over fifty meters from the valley floor, with one rock formation reaching down twelve metres and the other reaching down thirty metres. The twelve meter one is called "cascada chica" (small waterfall) and "flows" off a base which is about sixty metres wide. The other, "cascada grande" (large waterfall), extends down from a base with which is about ninety metres wide and eighty metres above the valley floor.

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.