Parque Nacional dos Lençóis Maranhenses

The name of this 598 sq mile (1550 sq km) national park refers to its immense expanses of dunes, which look like lençóis (bedsheets) strewn across the landscape; they stretch 43.4 miles (70km) along the coast and up to 31 miles (50km) inland. Located halfway between São Luís and the Piauí border, it's a spectacular place, especially from about May to September when rain filtered through the sand forms thousands of crystal-clear pools and lakes between the dunes. The lagoons are at their best in June, July and August. The park also includes beaches, mangroves and some interesting fauna, especially turtles and migratory birds.