The Lencois Maranhenses National Park ( Parque Nacional dos Lençóis Maranhenses ), composed of large, white, sweeping dunes, at first glance it looks like an archetypal desert, but lying just outside the Amazon Basin, the region is subject to a regular rain season during the beginning of the year that causes a peculiar phenomenon - fresh water collects in the valleys between sand dunes is prevented from percolating down by a layer of impermeable rock which lies underneath the sand - the resulting blue, green and black "lagoons" are surrounded by the desert-like sand, and reach their fullest between July and September - Maranhao state in northeastern Brazil.