Lençóis Maranhenses is a field of white sand dunes covering 1,550 square kilometres of the coast of Maranhão, in northeastern Brazil, and it is no desert: for half the year the rain fills the hollows between the dunes with thousands of clear blue lagoons that you swim in. The name means the bedsheets of Maranhão, for the way the dunes look from the air, white and rippled to the horizon, and the trick of the place is the water table: the rains from January to June soak the sand, the hollows brim, and from about June to September the lagoons are at their fullest, up to three metres deep, fresh rainwater over a floor of pale sand, warm at the edges and startlingly cold where it deepens; then the sun drains them, and by the year's end the valleys shrink to empty sand. Fish swim in the lakes every wet season although each dries up completely, hatched from eggs that survive the dry months in the sand or carried in by birds.
The field is crossed on foot with a guide, because there are no paths and no landmarks and the shape of the dunes is what the guide reads to lead you from one lagoon to the next, barefoot over ridges of warm sand. You push up the soft face of a forty-metre dune, sinking to your ankles, and from the crest the lagoons appear below in a chain, each a different blue in a white bowl; then you drop down and swim, and climb the next. Beyond the lagoons a stretch of pure sand runs on with no plant, no sound and no marker in any direction. The gateway is Barreirinhas, and 4x4 trucks cross the river and the sand to the edge of the field for the afternoon, when the light is low and the water warm; longer treks cross the whole field over days, sleeping in fishing villages on the far side. Lençóis Maranhenses is a desert that fills with lakes, and the lakes have fish in them.