Hierve el Agua means the water boils, and nothing here is hot. In the sierra east of Oaxaca city, cold springs come up on a clifftop at twenty-two to twenty-seven degrees, so charged with gas that they fizz as if they were on the boil. Nothing falls here either. The water carries more dissolved limestone than it can hold, so where it goes over the edge and dries, the mineral stays. What hangs off the cliff is a waterfall made of stone: the cascada grande, some thirty metres of pale ribbed rock shaped like falling water, with the smaller cascada chica beside it. A path drops from the plateau to its foot, where the whole curtain stands over you.
The top of the cliff is a flat shelf of the same mineral rock, and it is where the springs surface. They collect in shallow basins along the rim, one at the very lip, and the water is silky and heavy and stays cool against your skin under the Oaxacan sun. At the outer edge of that one the pool seems to stop in mid-air, with the valley of agave and cactus dropping away under you. At first light the basins go dead still and hold the sky, the ridgelines and the single bare branch in the shallows, until the first breeze or footstep breaks it. Almost all the rock under your feet up here was laid down by this water — about ninety-five per cent of the surface is stone the springs left behind. The Zapotec were using these springs long before anyone came to look at them. Canals cut into the rock as much as 2,500 years ago carried the water to terraced plots below, the only lined system of its kind known from ancient Mesoamerica. The same water that built the cliff coated those channels as it ran, so the canals themselves turned to travertine, and the faint stone traces of them still cross the plateau.