The Great Artesian Basin: Ancient Water Beneath the Outback

Imagine a vast hidden reservoir stretching beneath the heart of a continent – an underground world where water that fell as rain in ancient times still flows slowly through rock. This is Australia’s Great Artesian Basin (GAB), the largest and deepest artesian aquifer system on Earth. In a land known for its harsh, dry interior, the Basin is a subterranean lifeline, storing enough water to fill Sydney Harbour 130,000 times. The story of the Great Artesian Basin is a tale written across geological timescales – from the age of dinosaurs to the present – resulting in an awe-inspiring natural wonder that continues to shape ecosystems and human life.

Born of an Ancient Sea: Geological Formation

Over 100 million years ago, during the Mesozoic era, the landscape of inland Australia looked utterly different. Rivers and forests covered highlands on the eastern fringe, while much of the interior lay beneath the shallow Eromanga Sea, an inland sea teeming with marine life. I recently made a video on this sea and you can find the link to that in the description and in the pinned comment down below. As this ancient sea transgressed and receded during the Cretaceous period, it left behind layer upon layer of sediment. Erosion from nearby highlands first laid down vast sheets of sand and gravel, which compacted into porous sandstone aquifers. Later, as the Eromanga Sea flooded the interior, it blanketed those sandstones under marine silt and clay, which hardened into impermeable rock. This marine sedimentary cap formed a confining layer– effectively sealing in the porous sandstone and the water it held, like a lid on a gigantic pressure cooker. The sea lasted for about 15 million years. And during that time, an enormous amount of sediment was laid down.

As the Great Dividing Range uplifted along Australia’s east coast eons ago, it tilted the layers of the Great Artesian Basin, giving the system its gentle slope. By the late Cretaceous (around 95 million years ago), the Eromanga Sea retreated for good, leaving a buried legacy: an immense multi-layered aquifer system. Sandstone beds – products of river deltas and beach sands of that vanished sea – now form the main water-bearing layers. Between these aquifers lie harder or finer-grained layers like shale and mudstone that act as aquitards, slowing the movement of water and keeping it under pressure. This layer-cake geology of alternating permeable and impermeable strata is the essence of the Great Artesian Basin’s structure, enabling it to store an astonishing volume of groundwater over geological timescales.

But let’s dive a little deeper into what this actually means. What the basin physically looks like, and how the water is stored. Unfortunately, there isn’t some massive subterranean labyrinth beneath Australia with a magical amount of water beneath our feet like I envisioned when I was a kid, so my strategy of digging a deep hole and reaching it wouldn’t have worked. I just wanted to swim in it, y’know? The reality of it is a little less fun. But still just as interesting. Let’s start with the simple version. This is what most people are told.

The geological structure of the Great Artesian Basin can be thought of as an immense natural sponge, with distinct layers each playing a crucial role in storing and protecting groundwater. At the very bottom lies ancient crystalline bedrock, a dense foundation that prevents water from seeping deeper into the Earth. This base is made up of ancient, hard bedrock that consists of mostly crystalline rocks like granite and metamorphic rock from earlier geological eras. This acts as the foundation or basement of the basin and it does not hold water. It just stops it from seeping deeper into the Earth.

Resting above this basement are thick layers of porous sandstone— the deposits from the ancient Eromanga Sea. These sandstone aquifers contain countless microscopic pores between grains, capable of holding immense volumes of water. This is where all the good stuff is. The water we pump today comes from these sandstone deposits that are filled with water. Above the sandstone layers are sequences of fine-grained, impermeable rocks such as shale and mudstone. These upper layers act like a tight lid, trapping groundwater under immense pressure, preventing it from freely rising toward the surface. This "sandwich" of permeable and impermeable rock creates ideal conditions for an artesian aquifer system, enabling water to remain confined underground for millions of years.

When water is accessed from these sandstone aquifers, the immense natural pressure initially forces it to the surface without any mechanical aid—a phenomenon known as artesian flow. Historically, drilling into these pressurized layers caused water to surge upward, creating natural flowing wells. Over time, with increased extraction and reduced pressure, some areas no longer see water rise naturally. In these cases, mechanical pumps, such as electric or solar-powered submersible pumps, are installed deep within boreholes to lift water directly from the sandstone. These pumps create suction, drawing water through the sandstone’s microscopic pores, slowly pulling groundwater toward the borehole. Through careful modern management—regulating bore usage, sealing bores properly, and balancing extraction with natural recharge rates—the Great Artesian Basin continues to sustainably provide water essential to life across Australia's arid heartland.

So this is the simple version, but, in reality, like most things in life, things are a little more complex. You might notice that beneath the artesian basin, there are other basins like the Cooper basin. So why doesn’t the water seep below and fill these basins? Great question. I’m glad you asked. Whilst it’s true that some sections of the Great Artesian basin are indeed sealed with a crystalline basement rock, Beneath the Great Artesian Basin lies a complex tapestry of older geological basins, each formed at different stages of Australia's ancient geological history. These deeper basins represent successive phases of geological activity and sediment deposition, stretching back hundreds of millions of years. While these basins are stacked beneath the Great Artesian Basin, groundwater does not flow downward into them. But why? Well, this separation occurs because each of these deeper basins is sealed by thick layers of dense, highly compacted sedimentary rocks such as shale, mudstone, siltstone, and coal seams. These sedimentary layers act collectively as impermeable barriers, preventing water from penetrating into the deeper structures. Additionally, these deeper basins often contain significant deposits of hydrocarbons—oil and natural gas—which form under conditions that require complete isolation from water above. Because the sedimentary rocks separating each basin have extremely low permeability, groundwater in the porous sandstones of the Great Artesian Basin remains trapped, flowing horizontally or upward rather than downward. Thus, these deeper basins—ancient, sealed, and tightly compacted—remain geologically distinct from the freshwater aquifers above, maintaining separate geological identities despite their proximity beneath Australia's surface.

So with all of this being said, the result is a pressurized basin – an underground water world forged by the interplay of ancient rivers and seas and subsequently sealed beneath the outback plains.

A Massive Underground Reservoir: Size and Structure

The sheer size and scale of the Great Artesian Basin are difficult to fathom. Spanning 1.7 million square kilometers (about one-fifth of Australia), it underlies almost a quarter of the continent – covering 22% of Australia’s land area across Queensland, the Northern Territory, New South Wales, and South Australia. To put this in perspective, the Basin’s area is roughly the size of Texas, California, and Montana combined, or over 3 times the size of France. In places it reaches depths of 3,000 meters (3 km) underground, with multiple stacked aquifers existing.

Yes you heard that right, multiple stacked aquifers exist. Another nuance of this complex system. Let’s quickly dive into why. Sandstone layers in the Great Artesian Basin are stacked because they were deposited repeatedly over millions of years by rivers, deltas, and coastal environments as sea levels rose and fell. Each distinct sandstone layer marks a separate cycle of deposition, compaction, and solidification into rock, creating the basin’s characteristic layered geology. So the sandstone layer holding the water doesn’t just exist at the bottom of the basin. It exists in stacked layers. Each layer of sandstone holds water, separated by impermeable sedimentary layers like shale and mudstone. These stacked aquifers can occur at varying depths, some quite shallow and others very deep

The Great Artesian Basin's dimensions and capacity are truly immense, spanning an astonishing area of around 1.7 million square kilometers—equivalent to approximately 660,000 square miles. In places, the basin reaches depths of up to 3,000 meters (about 9,800 feet). Its vast sandstone aquifers collectively store an estimated 64,900 cubic kilometers of groundwater, roughly 65,000 trillion liters—a volume large enough to fill Sydney Harbour 130,000 times over. Remarkably, the age of water in this basin varies greatly: from freshly recharged rainfall along its eastern margins to ancient groundwater nearly two million years old in the basin's farthest reaches, making it a vast subterranean reservoir that truly bridges geological eras.

The Basin stretches from the Great Dividing Range in the east to deserts in the west, hidden below the ground yet crucial to life above it. For many remote regions, this ancient aquifer has long been the only reliable source of fresh water in an otherwise arid landscape.

Slow Journey of Water: Hydrology and Flow

How does water get into this underground giant, and where does it go? The hydrology of the Great Artesian Basin is a story of patience and time. Rainfall in distant mountains and highlands provides the primary recharge. Along the Basin’s elevated eastern margins – for example, the western slopes of the Great Dividing Range – rainwater soaks into exposed aquifer rock. These recharge zones act as entry points where water percolates down into the porous sandstones. Only a small fraction of rainfall makes this journey, filtering slowly through soil and rock, but over thousands of years even this small trickle has filled the Basin with an immense volume of water.

Once underground, the water begins a remarkably slow flow westward and southward. Gravity nudges the groundwater from the higher recharge areas and it seeps through the sandstones toward lower elevations deep in the continent’s interior. The rate of flow is almost glacial: on the order of meters per year. In fact, scientists estimate typical groundwater flow speeds of only about 1 to 5 meters per year in the aquifers. At such a crawl, a drop of water that enters the ground near the Basin’s edge might take tens of thousands of years to move just a few kilometers.

Over the vast distances of the Basin, groundwater that enters in Queensland or New South Wales may travel for hundreds of kilometers underground. By the time it reaches natural outlets in South Australia, that water can be extremely old. Isotope dating techniques (using carbon-14 and chlorine-36) reveal groundwater ages ranging from several thousand years near recharge zones to nearly 2 million years in the south-western discharge areas. In other words, some water emerging from springs today fell as rain before modern humans even existed, and even when Ice Age megafauna roamed the Australian plains. The Great Artesian Basin is essentially a time capsule, its waters carrying a memory of ancient climates.

Despite this slow movement, the system is dynamic on long timescales. Water is continuously (if slowly) added by recharge and lost by natural discharge. Recharge rates are extremely low compared to the volume stored – which is why the Basin’s water is often called “fossil water.” In many parts of the GAB, current extraction by wells exceeds the natural recharge, meaning the resource must be carefully managed to avoid depleting pressure. Essentially, tapping the GAB is like mining water that accumulated over millennia.

Springs and Oases: Water’s Return to the Surface

After traveling underground for ages, the water of the Great Artesian Basin finds its way back to the surface at certain special places. Natural artesian springs – often referred to as mound springs – occur where water under pressure escapes through cracks or thin spots in the confining layers. These springs are most abundant around the edges of the Basin or along geological faults. In the arid expanse of central Australia, an artesian spring is a little miracle: a steady flow of water creating a wet oasis amid the desert.

This is the mound spring in South Australia. Artesian pressure forces water up through the ground, depositing minerals that form a mound around the spring outlet. Unsurprisingly, mound springs get their name from the characteristic mounds of sediment (consisting of mainly calcium carbonate and sand) that build up over thousands of years as mineral-rich water flows out and evaporates, leaving behind deposits. They may appear as low, white-domed hills or raised ponds on an otherwise flat plain. Each spring is the visible tip of a metaphorical iceberg – a point where the deep groundwater resurfaces after its slow subterranean voyage.

Where these springs emerge, life flourishes. Many artesian springs support lush vegetation halos – green belts of reeds, grasses, and trees sustained by the permanent water supply. In stark contrast to the surrounding dry landscape, the spring wetlands create a habitat for a host of species. Fish and snails found nowhere else live in the spring pools, evolved in isolation over millennia. Tiny mollusks, crustaceans, and unique aquatic insects thrive in the warm mineral waters. Birds and mammals travel miles to drink from or live around these watering holes. There are over 90 plant and animal species that exist only in the Great Artesian Basin’s spring ecosystems– many of them in just a single spring complex. For instance, Dalhousie Springs in far northern South Australia alone, harbors at least 16 endemic species, including several small fish found only in its waters. Such springs are truly ecological islands, each with its own unique community adapted to the constant water supply. The main pool at Dalhousie is fed by warm (≈40°C) water, supporting dense vegetation in the middle of the desert. You might be curious about the warm temperature of the water? If so, I totally understand. While not all artesian springs fed by the basin emerge warm, the ones that do indicate a deep source, where water has absorbed heat from the Earth's interior, (a process known as geothermal heating) over thousands of years. By the time the water reaches the surface at Dalhousie Springs, it has already warmed significantly from prolonged exposure to the deeper, naturally heated rocks beneath the basin. The Dalhousie Springs originates from depths exceeding 1,000 to 2,000 meters. Now you might be thinking why does water escape here? When the pressurized water in the basin encounters geological weaknesses—such as faults, fractures, or thinner sections in the impermeable layers—it escapes upward, flowing to the surface naturally without pumping.

Sadly, some springs have diminished or fallen dry in the past century due to declining artesian pressure, which has been linked to excessive tapping of the basin. Dozens of free-flowing artesian bores drilled by early settlers gushed unchecked for decades, wasting water and reducing pressure. This drop in pressure caused several springs to cease flowing, threatening the specialized species that depend on them.

An Awe-Inspiring Legacy

And thus we reach the end of the remarkable story of Australia’s Great Artesian Basin. From rainfall seeping gently into mountain recharge zones, through its slow, patient journey within porous sandstone aquifers deep beneath the surface, to the life-giving springs that emerge to sustain thirsty wildlife in the harsh outback, the Basin embodies one of nature’s most extraordinary water cycles. This ancient underground reservoir, confined by layers of sedimentary rock, is not only an invaluable natural asset—it’s a geological miracle deserving of careful stewardship. Protecting it ensures this hidden reserve will continue to yield its liquid treasure, sustaining the continent’s unique ecosystems as it has for millions of years.

Here is the video we made on The Great Artesian Basin: