Two words - Lençóis, Brazil.
The Lençóis Maranhenses National Park (Parque Nacional dos Lençóis Maranhenses) looks like a desert. But it isn't. According to Wikipedia, it's an area of low, flat, occasionally flooded land, overlaid with large, discrete sand dunes. It encompasses roughly 1500 square kilometers, and despite abundant rain, supports almost no vegetation.
It is located in Maranhão state, in northeastern Brazil, just east of the Baía de São José. Composed of large, white, sweeping dunes it 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, spotting the desert with blue and green lagoons that reach their fullest between July and September.
You could have your very own lagoon, for a limited time offer.
The area is also surprisingly home to a variety of fish which, despite the almost complete disappearance of the lagoons during the dry season, have their eggs brought from the sea by birds.