Brazil Has a Sea of White Dunes That Fills With Blue Lagoons

Fly over the north coast of Brazil, in the state of Maranhão, and the ground below seems to have been covered with a freshly shaken bedsheet. White dunes roll away in long soft folds, and in every dip between them lies a pool of clear water, blue in one hollow and green in the next. Visitors walk barefoot over warm sand and then wade straight into a lake. The name says what it looks like: Lençóis Maranhenses means the bedsheets of Maranhão.

This is the largest coastal dune field in South America. According to NASA's Earth Observatory, the dunes cover about 900 square kilometres, roughly the area of New York City, and some of them rise 30 metres. They are made of white quartz sand, so bright that from above the lagoons look like pieces of sky that have dropped in. In 2024 the park was added to UNESCO's World Heritage list.

What people often get wrong

Almost everybody calls it a desert, and it certainly dresses like one. But a desert is a place where very little rain falls, and here about 125 centimetres come down every year. NASA points out that this is slightly more than Seattle gets and double the rainfall of London. The dunes are not there because the land is dry. They are there because there is a great deal of sand and a great deal of wind.

Where the sand comes from

The sand arrives by water. Rivers, the Parnaíba among them, deliver quartz sand in enormous quantities to an unusually flat stretch of coast. Then the wind takes over. Steady trade winds blow in from the east, and in the dry season they reach at least 50 kilometres an hour, heaping the sand into dunes and pushing those dunes west by roughly 4 to 25 metres a year. So the bedsheet is never quite the same shape twice.

• 125 cm of rain a year, slightly more than Seattle gets
• 30 m the height some of the dunes reach
• 900 km squared, the dune field, roughly the area of New York City
• 4 to 25 m how far the wind pushes the dunes west each year

How the lagoons hold their water

Rain falling on a sand dune ought to sink straight through it. Much of it does, but under the sand lies a layer of bedrock and clay that lets very little water pass. In the rainy season the water gathers above that layer and shows itself in the hollows between the dunes as lagoons. They are at their fullest around June. National Geographic has reported pools more than 90 metres long and up to 3 metres deep.

Then the rain eases off. Between August and November the water table sinks, and by December most of the lagoons have dried away, leaving pale sand where people were swimming a few months before. Not everything leaves with the water: one fish found here, the wolffish (Hoplias malabaricus), spends the dry season dormant in damp mud and comes out again when the pools return. NASA notes that more than 850 species have been recorded in the park.

So the place is a puzzle with a simple answer. It has too much rain to be a desert and too much sand to be anything else, and for a few months each year it manages to be a dune field and a lake district at the same time. Then the wind gives the sheet another shake, and the whole thing starts again.