Lençóis means "sheet" in Portuguese, and it's a pretty adequate name...it's a desert with an endless number of what appear to be white sheets on the horizon, with whiter sand and bluer skies than I've ever seen in my life. The most incredible part is that in between the dunes, there are lagoons of fresh rain water which are surprisingly cool and refreshing.
The Lençóis Maranhenses are a National Park and private vehicles are no longer allowed. You get there on ATVs, normally Toyotas. You have to cross some of the lagoons and the water gets inside the car, so make sure to put your camera in a plastic bag!