Hierve el Agua (the Water Boils) is a state park located 80km/48mi (about 2 hours) southeast of Oaxaca City.There is a collection of pools at the site fed by mineral springs that flow at about 2 liters/second. Two waterfalls with beautiful calcium deposits drop over cliffs in the Sierra Madres at an altitude of 5800'.
The springs were used in a Zapotec irrigation system dating back to around 1300 B.C. The water has a high calcium carbonate and magnesium content that has resulted in the heavy deposits seen in the photo at right. The water bubbles with escaping air giving the appearance that it is boiling. It is actually a warm spring with a temperature in the 71-77°F range. You can go for a swim in the pools.