The Poza Azul is the most accessible and most famous of the hundreds of spring-fed pools that dot the Cuatrocienegas valley. The name is straightforward — the water is a startling blue-green, perfectly clear, and fed by underground springs that maintain a constant temperature year-round. The pool sits in the middle of the Chihuahuan Desert with no obvious source, which adds to the sense of unreality.
Swimming in the Poza Azul is permitted and the experience is worth it — the water is clean and cool, and looking down through it you can see the sandy bottom and the edges of the spring where the water bubbles up. Entry to the pool area requires a permit from the Biosphere Reserve visitor centre in town; numbers are limited to protect the ecosystem. Do not use sunscreen or insect repellent in or near the water — the chemical contamination can harm the endemic species.