The sand is not local. The Parnaíba and the Preguiças carry it down to the Atlantic coast of Maranhão, where the surf works it up onto the beach and the prevailing wind takes over, pushing it as far as fifty kilometres inland. The result is a dune belt running along seventy kilometres of coastline and extending five to twenty-five kilometres back from the sea, with individual dunes reaching forty metres. From the air the pattern reads as long parallel ridges, which is where the name comes from — lençóis, bedsheets, laid out to dry. The whole field is in slow motion. Every dry season the wind edits it, and the map of the lagoons redraws itself accordingly.

Deserts, by the standard definition, receive under 250 millimetres of rain a year. This place gets about 1,200, roughly seventy percent of it between January and May, which disqualifies it from the category it so obviously resembles. What makes the lagoons possible is not the rain alone but what lies beneath the sand: a layer of impermeable rock that stops the water draining away. It collects in the interdune hollows instead, in lagoons up to a hundred metres long and three metres deep, covering something like forty-one percent of the park at their fullest. The water is warm — between 27.5 and 32 degrees Celsius — and distinctly acidic, with a pH between 4.9 and 6.2. It is also, being rainwater filtered through clean sand, startlingly clear.

Surveys have recorded 133 plant species, 112 birds and at least 42 reptiles here, along with four threatened species: the scarlet ibis, the neotropical otter, the West Indian manatee and a small wild cat. The organism that best explains the place, though, is a fish. Hoplias malabaricus, the traíra, survives the dry season by burrowing into the mud at the bottom of a lagoon and waiting — months, sometimes, in the dark, until water returns above it. How fish come back to pools that demonstrably vanished is one of the first questions visitors ask, and the mud is most of the answer. Colonising a landscape that deletes itself every year takes a particular kind of patience.

Federal protection arrived on 2 June 1981, under Decree 86,060, covering roughly 155,000 hectares — Brazilian figures put the area at 156,584 — across the municipalities of Barreirinhas, Primeira Cruz and Santo Amaro do Maranhão. About 90,000 hectares of that is open, unvegetated dune. The park is administered by the Chico Mendes Institute for Biodiversity Conservation, and the boundary was drawn around more than scenery: it takes in beach, restinga, mangrove at the river mouths, and the handful of small communities living inside the dune field. Queimada dos Britos and its smaller neighbour Baixa Grande sit in oases several hours' walk apart, on the trekking line between Atins and Santo Amaro. Motorised transport inside the park is banned; only registered residents may run quad bikes, so everyone else arrives on foot or on an animal. Baixa Grande was down to about thirteen families in 2017. The children go to school in Barreirinhas, and most of them stay there.

In July 2024, forty-three years after the decree, UNESCO inscribed Lençóis Maranhenses on the World Heritage list as a natural property, citing its exceptional beauty and the fact that the phenomenon it protects is found nowhere else on earth. The recognition ratified something the visitor numbers had already established. Older sources describe a park receiving perhaps 60,000 people a year; Brazilian figures for 2023 put arrivals above 400,000. Some of that traffic followed the cameras — the dunes stood in for the planet Vormir in Avengers: Infinity War and Avengers: Endgame, and long before that they were the setting for the Brazilian film The House of Sand. Whether a landscape that erases and rebuilds itself every year can absorb visitor numbers of that order is now the park's central management question, and the answer is not yet in.