Olympic trainees take note: You can practice your butterfly just about anywhere—even when surrounded by desert sands. Of course, that's only if you're in Mexico's Cuatro Ciinegas Reserve. Declared a protected area in 1995, the reserve's labyrinth of underground rivers has produced a large number of aquatic pools (many more than the "four lagoons" that give the town its name). Swimming is permitted in only a few of the lagoons to protect the local flora and fauna, and because of the environmental fragility, avoid using suntan lotion or any kind of soap when you're soaking. After your swim, climb up the limestone sierras for a spectacular view of the valley.