Hierve el Agua is one of Oaxaca's most striking natural formations: mineral springs on a cliff edge about 70 km east of the city, where calcium and magnesium carbonate deposits have built up over millennia into formations that look exactly like frozen waterfalls pouring off the rock face. Two natural pools — one large, one small — overflow at the cliff edge, and the views across the Cañada valley are dramatic.
You can swim in the pools (not hot, but not cold either — around 20°C) and walk down a steep path to see the petrified cascade formations from below. The site is best visited on a weekday; weekends bring significant crowds from Oaxaca. From the city, take a colectivo to Mitla and a second colectivo from there — about 2.5 hours each way. A day trip combining Hierve el Agua with Mitla, Yagul, and the Tule tree covers the essential valley circuit.