Hierve el Agua, Mexico, Oaxaca: A rare natural phenomenon

What boils and is cold enough to touch? What flows and is motionless? What is ancient but constantly renewing? What freezes in blazing heat? Solve the riddle and you will arrive at a sacred place – the petrified waterfalls of Mexico's Hierve el Agua.

Its name means "the water boils" and while it does indeed bubble to the surface in the pools that feed the falls, it is cool – between 22 to 25 degrees. These two "waterfalls" set in the high, remote mountains of the southern Oaxaca region, seem to tumble into the valley below, the majestic drops clearly defined. Yet they are solidified torrents, frozen to resemble Arctic ice waterfalls.

Hierve el Agua's natural rock formations are formed by freshwater springs saturated with calcium carbonate and other minerals. As the water flows over the cliff, the minerals are deposited to form solid falls, much like stalactites.

Little was known of the Cascadas de Hierve el Agua until the 1994 US FINA World Cup when Mexico's Corona beer used the frosty white waterfalls in a video advertisement to replicate an overflowing head of beer. To visit this rare natural phenomenon – there's another in Pamukkale, Turkey – requires a two-hour journey south-east from Oaxaca City, the last half hour or so on a rough mountain road.

Historically, this is indigenous Zapotec country. Their pre-Colombian, 2500-plus-year civilisation flourished in Mesoamerica's Valley of Oaxaca. Mesoamerica was a geographic region that prospered before Spanish colonisation and extended from central Mexico through Belize, Guatemala, El Salvador, Honduras, Nicaragua and northern Costa Rica.

Hierve el Agua, at 1821 metres in the heart of the Sierra Madre del Sur, was sacred to the ancient Zapotec. Its water nourished their crops within a mountainous region of extreme seasonal aridity. They built a complex irrigation system here, an extensive webbing of lined canals and terraces cut into the sides of the mountain cliffs. The irrigation system is considered unique in Mesoamerica and its vestiges continue to interest archaeologists, biologists and geologists. Irrigation was unusual in pre-Hispanic Mexico.

It's hard to imagine that these scorched mountains marching towards an Oaxacan horizon are green-furred during the May to November rainy season. During our March visit, the heat and dust shimmer ruthlessly.

Still, our group is keen not simply to observe the admittedly remarkable spectacle of the blue-green pools and calcium carbonate deposits that step down the mountain, but also to hike the circular one-hour loop to properly appreciate the two petrified waterfalls – the 12-metre cascada chica (small cascade) and the 30-metre cascada grande.