Who said desert oases don't exist? Lençois Maranhenses National Park is the only desert in the world with thousands of crystal clear freshwater lagoons. During the first half of the year, the sand dunes fill with water from the extensive rainy season. Then, during the dry season many of them disappear. Covering over 383,000 acres in the Maranhão State of Brazil, sand from riverbeds has contributed to the formation of the park over the past several thousand years.
The best time to visit is right after the rainy season (roughly May to June) when you'll find the lagoons at their fullest and the weather enjoyable.
If you're making the trip all the way out to the dunes, you should definitely get a guide to drive you through sections of the park in a jeep. Stop and take a dip in one or two of the lagoons, or even fly over the park for some spectacular panoramas.