Cuatro Cienegas

Cuatro Cienegas is a valley in the Chihuahuan desert where groundwater surfaces in hundreds of springs, pools and marshes - pozas of astonishing turquoise ringed by white gypsum and mesquite, isolated from other water systems for so long that they evolved their own biology. Scientists have described dozens of endemic species here, from fish and snails to turtles and crustaceans, and the pools contain living stromatolites, the layered microbial mats that dominated early Earth - which is why NASA-linked researchers have studied the valley as an analogue for early life. Visitors see it at a handful of managed sites: the Poza Azul viewing platforms, where swimming is prohibited and the blue is best photographed; the Rio Mezquites, where a float in clear water is allowed under rules; the Playitas pool; and the gypsum dunes, a field of blinding white crystals with a boardwalk. The pressure on the aquifer from irrigation is the valley's crisis, well documented and often signposted, which makes the visiting rules - no sunscreen, no soaps, no swimming outside designated spots - a matter of survival rather than etiquette.

What you actually see

• The blue pozas - turquoise springs in white gypsum ground.
• Living stromatolites - microbial reefs in the shallows.
• The gypsum dunes - a white crystal field with boardwalk.
• The desert valley - mesquite, mountains and sudden water.

When to go

October to April for tolerable desert heat; mornings for the strongest colour in the pools. Summer is punishing and some sites restrict access.

Honest expectations

Most pools are look-not-swim, and rightly so; only designated spots allow water contact. Sunscreen and repellent are banned in the water everywhere. Facilities are simple and distances between sites need a car.

Nearby

Cuatro Cienegas town's wine and goat-sweet traditions sit at the valley's centre; Monclova and Saltillo lie east.