Guatin is a narrow canyon carved into the desert about 30 kilometres from San Pedro de Atacama. What makes it unusual is the life inside it: a stream runs along the bottom, feeding tall cardón cacti and desert shrubs that have no business growing at this altitude. The trail follows the watercourse between high volcanic walls, climbing gently over rock and sand. You hear the water before you see much of anything, and the deeper you go, the greener it gets. The trek is moderate and suitable for most fitness levels. Your guide points out the geology, the plants, and the birds that nest in the canyon walls. It is one of the Atacama's quieter walks, and one of the most surprising.