From Wikipedia:
The Lençóis Maranhenses National Park (Parque Nacional dos Lençóis Maranhenses) is a national park located in Maranhão state, in northeastern Brazil, just east of the Baía de São José, between 02º19’—02º45’ S and 42º44’—43º29’ W. It is an area of low, flat, occasionally flooded land, overlaid with large, discrete sand dunes. It encompasses roughly 1,500 km2 (580 sq mi), and despite abundant rain, supports almost no vegetation. The area became a National Park on June 2, 1981.
Composed of large, white, sweeping dunes, at first glance Lençóis Maranhenses looks like an archetypal desert, but in fact it is not an actual one. Lying just outside the Amazon Basin, the region is subject to a regular rain season during the beginning of the year. The rains cause a peculiar phenomenon: fresh water collects in the valleys between sand dunes and 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.
The lagoons have large numbers of fish that arrive when the lagoons are at their fullest after July, when they are interconnected to rivers such as the Rio Negro. One species of fish, the wolf fish or tiger fish (Hoplias malabaricus) stays dormant in the mud and moist areas after the majority of the water has evaporated, re-emerging during the next rainy season.
