With large, white, sweeping dunes, Lençóis Maranhenses looks like a typical desert, but in fact it is not one. Being just outside the Amazon Basin, the region gets a regular rainy season in first half of the year. When it rains, fresh water collects in the valleys between sand dunes. The water is prevented from sinking down by a layer of rock underneath the sand. This results in blue, green and black lagoons surrounded by the desert-like sand. The lagoons are at their fullest their fullest between July and September.