Why This Desert Fills With Thousands of Lakes Each Year: Lençóis Maranhenses

A Place That Shouldn’t Exist

From above, it doesn’t make sense.
Stretching along Brazil’s northeastern coast, the landscape looks like a frozen ocean — wave after wave of white dunes rolling toward the horizon, bright as powdered glass. But then you notice something impossible: between those dunes, thousands of turquoise pools glint in the tropical sun, calm as gemstones. A desert… filled with lakes.

This is Lençóis Maranhenses National Park, one of the strangest and most beautiful landscapes on Earth. The name means “The Bedsheets of Maranhão,” and it fits — the dunes look like rippled sheets tossed by some giant hand. But what really makes this place a wonder is the contradiction at its heart. It looks like a desert, yet it rains more than London. It’s a place where wind and water take turns sculpting the land — a place that breathes.

Each year, during the dry months, the Atlantic trade winds sweep across the coast, pushing sand inland and reshaping the dunes. Then, when the wet season hits, the rain arrives in a dramatic rhythm, filling the hollows between dunes with water so clear and blue it seems unreal. For half the year, the desert becomes a temporary archipelago — a living mosaic of shifting sand and shimmering lagoons.

It’s this delicate choreography between wind, rain, and geology that makes Lençóis Maranhenses so rare. And to understand how such a place can exist at all, we need to dig into its story — a story written in sand, water, and time.

*Picture depicts the Lencois Maranhenses National Park 

The Ancient Stage Beneath the Sand

The story begins long before the dunes ever existed — millions of years ago, when sea levels along this stretch of the Brazilian coast were rising and falling like a slow geological tide. Each time the ocean advanced, it spread sand across the lowlands. Each time it retreated, rivers carved through those sediments, laying down new layers.

Over countless cycles, these shifts built up a wide coastal plain — part of what geologists call the Barreiras Formation, a mixture of old river and coastal deposits: sandstone, silt, and clay. Think of it as a layered canvas, waiting for new forces to start painting.

Fast forward to the last Ice Age. Sea levels dropped, exposing wide beaches and coastal flats. When the glaciers melted again, the sea returned, pushing those deposits inland, reworking and redistributing the sand. The result was a huge supply of fine quartz sand, ready to be moved. All it needed was a little help from the wind.

And that’s exactly what happened.

The Sand, the Sea, and the Wind

The sand that makes up Lençóis Maranhenses has two parents — rivers and the ocean — and one sculptor: the wind.

While the nearby Preguiças River delivers sediment directly to the coastal plain adjacent to the dune field, the larger Parnaíba River delta to the east may also play a supplementary role by supplying sand that is carried westward by longshore currents along the Maranhão coast

Once the sand hits the shoreline, it doesn’t stay put. The waves and tides shuffle it east and west, piling it along beaches and flat coastal plains. From there, strong onshore trade winds — those same winds that make Brazil’s northeast famous for kitesurfing — pick up the grains and drive them inland.

Over time, the wind sculpts the sand into sweeping ridges called barchanoid dunes — long, crescent-shaped dunes that align in ribbons running inland for tens of kilometres. Between these ribbons are troughs, low valleys where the wind has scooped away sand. Those troughs will soon become something magical.

You can see the process from space: dunes migrating, merging, reshaping year after year. Satellite surveys and 3D elevation models show that the dunes can move several metres each season. Some gain height, others collapse. It’s not a still landscape — it’s one that’s constantly rewriting itself.

So, the sand keeps moving, the wind keeps sculpting, and the dunes keep marching inland. But all this movement alone can’t explain the lagoons. For that, we need to talk about water — and a secret lying just beneath the surface.

The Desert That Out-Rains London

It sounds impossible, but Lençóis Maranhenses receives around 1.3 to 1.5 metres of rainfall each year — nearly double what London gets. And the difference isn’t just in volume, but in rhythm. Here, nearly all that rain falls during the first half of the year.

When the wet season arrives, the clouds roll in from the Atlantic and unleash downpours over the dunes. For a few months, this “desert” becomes one of the wettest places in Brazil’s north. The rain seeps down through the sand, soaking it like a sponge. But it doesn’t just disappear — because underneath, something special happens.

The water table — the upper level of groundwater beneath the surface — rises sharply during the rains. Imagine a hidden underground tide slowly lifting upward. When it meets the low points between dunes, the water breaks through to the surface, filling the hollows.

That’s what creates the lagoons: not rivers or springs, but the groundwater itself becoming visible. Each lagoon is like a window into the water table. When the dry season returns, the water table sinks again, the lagoons shrink and vanish, and the wind resumes its work, reshaping the sand once more.

This rise and fall — this breathing of the land — is what gives Lençóis Maranhenses its living rhythm.

 *Image depicts the water table rising to the level of the Lagoon.

*Image depicts water being "trapped" by the non-porous layer of clay and silt. The brown line above the clay/silt layer depicts the organic material that acts like a plug.

Why the Water Doesn’t Vanish

Here’s the question that stumps everyone who visits: if it’s all sand, why doesn’t the water just drain away?

The secret lies in the layers beneath the dunes. The upper blanket of sand is extremely porous — water can move through it easily. But below that are older coastal and river deposits left behind by those ancient sea-level changes. These layers contain clay and silt, which are much less permeable. Over years of wet seasons, thin films of mud and organic material have also built up at the bottoms of the lagoons, forming natural seals.

Together, these act like a patchwork tarp under a sponge: the water still seeps slowly downward, but not fast enough to drain the lagoons before the dry season ends. It’s a beautifully balanced system. If the layers below were too tight, the dunes might become swampy and lose their shape. If they were too loose, the water would vanish before it ever filled a lagoon.

Geologists studying Lençóis have confirmed this delicate structure — a blanket of clean, fine aeolian sand (sand moved by wind) resting on a more complex foundation of coastal and fluvial sediments. It’s this contrast in permeability that lets the lagoons persist for months.

The water itself is fresh and clear, slightly acidic from contact with the sand and organic material. Sunlight bounces off the white quartz grains below, giving the lagoons their brilliant blues and greens. The colours shift with depth, light, and the angle of the sun — a natural light show created by physics and geology.

The Landscape in Motion

Lençóis Maranhenses covers about 156,000 hectares — roughly the size of London — with nearly 90,000 hectares of active dunes. The dune chains can stretch for up to 80 kilometres inland, forming one of the largest coastal dune systems in South America. But it’s not static.

Scientists tracking dune migration with satellite imagery found that individual dunes can move several metres per year, sometimes more. A lagoon that’s chest-deep in one season might be gone the next, swallowed by shifting sand, while another forms a few hundred metres away. It’s a living, moving system — not a monument frozen in time, but an ongoing performance between wind, water, and gravity.

During the height of the rainy season, the lagoons often link up, forming corridors of water that stretch for kilometres — chains of turquoise beads strung between ivory ridges. By the end of the dry season, most of those links break apart, leaving scattered ponds that fade into damp sand.

This pulse — filling, shrinking, filling again — has been repeating for thousands of years. It’s so regular that the dunes and water table have actually synchronised. Studies modelling the system show that the period of groundwater rise and fall matches the migration pace of the dunes. The land and its water are literally in step — the dunes move just fast enough that the lagoons reappear in the same rhythm each year.

The Perfect Balance

You can think of Lençóis Maranhenses as a machine with four great gears, all turning in harmony.

The first gear is sand supply — rivers feeding the coast with fresh material. The second is wind, blowing hard and steady enough to organise the dunes into neat ridges instead of chaotic piles. The third is rain, arriving in powerful bursts that lift the water table. And the fourth is the ground itself, with its subtle layers that hold water just long enough to form lagoons but not so long that they stagnate.

Then there’s the fifth, finer gear — timing. The dunes’ migration pace and the seasonal rhythm of the rain are perfectly aligned. Get any of these gears wrong — a weaker wind, a drier season, or a leakier base — and the whole system would unravel. Too dry, and you’d have a lifeless desert. Too wet, and the dunes would drown under permanent lagoons.

This is why Lençóis Maranhenses is so rare. It’s not just the presence of dunes and rain — it’s the exact balance between them.

A Living System, Not a Monument

Every year, nature rewrites the script. Dunes collapse, lagoons form anew, and the pattern shifts just enough to remind us that this landscape isn’t frozen — it’s alive.

It’s also why theories of catastrophic origins, like ancient tsunamis, don’t hold up. A tsunami would leave behind chaotic debris: pebbles, shells, and unsorted sediment dumped in a single violent event. But what we find here is order — clean, wind-sorted sand, gentle layering, and ongoing motion. Lençóis wasn’t created in an instant. It’s still being created, season after season, breath by breath.

From a geologist’s point of view, this place is a masterpiece of equilibrium — the kind of natural system you’d expect to collapse under its own contradictions, yet somehow it endures. From a traveller’s point of view, it’s pure magic — a desert that refuses to stay dry.

The Wonder of Balance

Stand on a dune at sunset and you can feel it — the slow heartbeat of the place. The wind brushes your face, the sand shifts under your feet, and down below, the lagoons mirror the fading light. They are not permanent, but that’s the point. Their beauty lies in their impermanence — in the balance between elements that shouldn’t coexist but somehow do.

Lençóis Maranhenses exists because everything here happens just right. The rivers deliver sand. The wind sculpts it. The rain fills it. The earth holds it. And together, they create a desert that’s anything but dry.

Here's the video we made on this on the OzGeology YouTube Channel: