The Lençóis Maranhenses — the bedsheets of Maranhão — is a national park of white sand dunes covering some fifteen hundred square kilometres of the northern coast. That alone would make it striking. What makes it one of the great landscapes of South America is the rain: from January the wet season fills the hollows between the dunes, and by mid-year there are thousands of lagoons of clear blue and green water lying in the sand.
The lagoons are at their fullest from roughly May to September, and July is the classic month. By December most have evaporated and the park is mostly dry dunes again. They are rainwater, warm and clean, and swimming in the Lagoa Azul or the Lagoa Bonita with nothing but sand in every direction is the whole point of the trip.
There are no roads inside the park. Most people base themselves in Barreirinhas, on the Rio Preguiças, and go in on four-wheel-drive tours or boat trips down the river to the coast. Atins, at the river mouth, is the quieter, more remote base, and a starting point for multi-day walks across the dunes to Santo Amaro, sleeping in fishing families' homes at the oases of Baixa Grande and Queimada dos Britos.
Take sunglasses, a hat and more water than you think: there is no shade anywhere.