The Flood That Left a Sea of Grass: South Australia's Lake Eyre and the Great Artesian Basin

On a map of Australia, the Lake Eyre basin looks like a wound—a pale, irregular stain the size of France and Spain combined, bleeding inland from the continent's heart. It is the lowest natural point in Australia, fifteen metres below sea level, and it is almost always dry. But the water that once filled it has not disappeared. It sank.

The Basin That Holds a Sea

Beneath the Lake Eyre basin lies the Great Artesian Basin, the largest and deepest groundwater system on Earth. It stretches across 1.7 million square kilometres—a fifth of the Australian continent—and holds enough water to cover the entire country in a shallow sea, had the rock not trapped it.

The water is ancient. Carbon-14 and chlorine-36 dating place most of it between 10,000 and 40,000 years old, with some pockets exceeding a million years. It moves at a geological crawl: a few metres per century, seeping through porous sandstone aquifers that were laid down during the Cretaceous period, between 145 and 66 million years ago.

The Rock That Became a Sponge

During the Cretaceous, eastern Australia was submerged beneath a shallow inland sea called the Eromanga Sea. Rivers from the rising Great Dividing Range carried sediment westward—sand, silt, clay—and deposited it in layers kilometres thick. When the sea retreated, it left behind the sedimentary rock that would become the Basin's reservoir.

The key formation is the Cadna-owie Formation, a quartz-rich sandstone that acts as the primary aquifer. Above it, the Bulldog Shale and other fine-grained units form a caprock that prevents the water from escaping. The Basin is a giant, tilted sandstone sandwich: the water enters where the rock outcrops along the western slopes of the Great Dividing Range, then travels slowly downward and westward under hydraulic pressure.

The Basin is not a lake underground. It is a fossil of a sea—a sea that evaporated, then soaked into the sand and stayed.

The Sink That Became a Salt Pan

Lake Eyre itself is the terminal sump of this vast system—the place where the Basin's pressure finally forces water to the surface, only for it to evaporate under a desert sun. When it does fill, which happens once or twice a century, it becomes Australia's largest lake. But even then, the water is rarely more than a few metres deep.

The lake bed is a salt crust up to half a metre thick, composed mainly of halite and gypsum. Beneath the salt lies a layer of dark, organic-rich mud that records the lake's wetter past: pollen from rainforest plants, bones of Diprotodon and other megafauna, and layers of silt that correlate with ice-age pulses in the Northern Hemisphere.

When the lake is dry, it reflects the sky like a mirror. Satellite images show a white oval that shifts shape with each flood event. The salt crust cracks into polygons, and the wind sculpts them into ripple patterns that look like the surface of another planet.

The Water That Ran Uphill

Before European settlers drilled into the Great Artesian Basin, the water emerged naturally at springs—mound springs, they are called—where the pressure forced it through cracks in the caprock. These springs are among the most ancient ecosystems on the continent. Some have been flowing continuously for more than a million years, supporting endemic fish, snails, and crustaceans that survive nowhere else.

The settlers drilled hundreds of bores, and the water gushed freely—sometimes at pressures high enough to shoot thirty metres into the air. For a century, the Basin was treated as inexhaustible. It was not. By the 1990s, flow rates had dropped by more than half, and many springs had dried up. Cap-and-bore programs have since slowed the decline, but the system is still being drained faster than it recharges.

The water that falls on the Great Dividing Range today will not reach Lake Eyre for tens of thousands of years. By then, the climate may have shifted again. The Basin is a memory of a wetter Australia, written in sandstone and moving at the pace of geology.