The Hidden Grand Canyon Beneath Perth

The Perth Grand Canyon

Just off the coast of Perth, beyond where the Indian Ocean darkens to cobalt blue, there lies a colossal scar cut deep into the seafloor — a canyon so vast it rivals the Grand Canyon of Arizona in scale. Yet almost no one on land has ever seen it. It’s called the Perth Canyon, and while it sits just fifty kilometres west of the city. You could live your whole life in Western Australia and never realize that one of the country’s largest geological features lies right offshore. The story of this canyon, however, stretches far beyond Perth’s skyline. It begins in the age of dinosaurs, during the time when Australia was still welded to India and Antarctica, and the continents were about to tear themselves apart.

To picture the Perth Canyon, imagine standing on a desert plain, except it’s submerged beneath hundreds of meters of water. Now carve into that plain a gorge that starts at about 180 meters deep — right at the edge of the continental shelf — and runs for 120 kilometers out to sea, plunging down to almost 4,500 meters. That’s a drop greater than Mount Kosciuszko is tall. The canyon’s flanks are steep and sometimes nearly vertical, with vast terraces and sharp scarps that catch the light in multibeam sonar images like cliffs on Mars. These walls are cut into ancient rocks that record hundreds of millions of years of Western Australia’s geological history — the last remnants of when this part of the continent was alive with tectonic energy.

*Image shows the Perth Grand Canyon

When Continents Split and Rivers Carved the Shelf

To understand how such a giant canyon came to be here, we have to rewind the geological clock roughly 140 million years, to the time when Australia was breaking away from India. Back then, the land that’s now Western Australia sat at the edge of a massive rift system. As the two continents began to drift apart, the crust stretched, fractured, and sank to form a long trough — what geologists call a rift basin. The Perth Canyon sits within one of these rifts, known as the Vlaming Sub-Basin, part of the larger Perth Basin. Faults — deep cracks in the crust — formed as the land stretched and thinned. Over millions of years, sediments filled these depressions: sandstones, mudstones, and limestones washed in from rivers and deltas that once drained the Australian interior.

*Image depicts Australia joined to Antarctica and India with a rift forming between India and Australia.

*Image depicts the Vlaming Sub-Basin

*Image depicts The Palaeo Swan River when sea levels were lower and the continental shelf was exposed. The pathway shown is for illustrative purposes only and does not necessarily correspond to the actual pathway followed by the ancient river.

At the time, the area wasn’t an ocean floor yet. It was a broad, shallow continental shelf that periodically rose and sank with changes in sea level. Rivers carved valleys into the soft sediments, shaping the groundwork for what would eventually become the Perth Canyon. One of these rivers, the ancient ancestor of today’s Swan River, likely played a starring role. When sea levels fell, the river extended far out across the continental shelf, cutting into it like a knife through butter, carrying its sediments and freshwater far beyond the modern coastline. When sea levels rose again, those valleys flooded, filled with marine muds, sands, and eventually limestone. Each fall and rise of the ocean left a new mark, creating stacked incised valleys — buried canyons within canyons — that geologists can still see today in seismic records beneath the seabed.

Fast forward to the Late Cretaceous, around 80 to 70 million years ago, and we find the Perth Canyon in its first major phase of activity. Australia had separated from India by then, and the western margin of the continent had gone quiet tectonically. The land was stable, arid, and gradually subsiding under its own weight. But this stability came with a twist: global sea levels began to drop dramatically, and that retreating ocean exposed the continental shelf once more. Rivers seized the opportunity, deepening their channels and carrying enormous volumes of sediment toward the ocean. The Swan River — or rather, its ancient, much larger predecessor — likely carved an enormous valley into the shelf, forming what scientists now identify as the first palaeovalley, known as PV1. It stretched nearly 12 kilometers wide and several hundred meters deep, and when sea levels rose again, it was buried beneath new marine sediments.

This process repeated itself multiple times throughout the Cenozoic Era — the last 65 million years. Each major drop in sea level would expose the shelf, allowing new rivers to cut smaller valleys into the same region, each one slightly offset and shallower than the last. By analysing seismic data and core samples, geologists have identified at least three of these stacked valleys — PV1, PV2, and PV3 — each recording a pulse of erosion during a different geological period. PV1 dates to the Late Cretaceous, PV2 to the early Oligocene (around 34 million years ago), and PV3 to the late Miocene (around 5 million years ago). Over time, these valleys were buried by new sediments, forming a kind of geological layer cake that traces the waxing and waning of the ocean across millions of years of Earth history.

A Canyon Reborn Beneath the Sea

But the canyon as we see it today wasn’t carved by a river — at least, not directly. The modern Swan River ends on land, more than fifty kilometers east of the canyon head, and there’s no connection between the two. Instead, the Perth Canyon is now considered a relict submarine canyon, meaning it’s a feature left over from an earlier time when the shelf was above sea level. Once submerged, the canyon continued to evolve under the influence of marine processes. Gravity-driven flows of sediment, called turbidity currents, occasionally roared down its flanks — undersea avalanches that carried mud, sand, and silt to the deep abyssal plains. Over time, these flows helped deepen and widen the canyon, sculpting its steep walls and creating depositional features known as cyclic steps — rhythmic ridges and depressions carved by the pulsing energy of turbidity currents. But unlike canyons fed by modern rivers, which receive a constant supply of sediment, the Perth Canyon is largely starved. Its slopes are stable, its fan deposits are thin, and turbidity currents are rare events today.

In the words of the scientists who mapped it using the research vessel Falkor, it is now a “fossil canyon.” The ROV footage taken during the 2015 expedition shows a quiet, almost eerie landscape. The walls rise hundreds of meters, layered with pale mudstones and chert. In some places, you can see traces of old slumps — where part of the wall sheared off and slid downward. Elsewhere, narrow gullies and overhangs are draped in a fine dusting of sediment that hasn’t moved in centuries. It’s a world frozen in time. Even two large earthquakes that struck Western Australia in 2018 failed to shake loose more than a few small slides near the canyon head. For a landform that once echoed with the roar of underwater landslides and sediment flows, the Perth Canyon today is astonishingly still.

The Stratigraphy and Tectonic Scars of the Canyon

The deeper layers of rock exposed in the canyon walls tell a story of Western Australia’s ancient environments. The oldest sediments, found near the base, date back to the Late Paleocene, around 56 million years ago, and consist of wackestones — carbonate muds that formed in a warm, shallow sea. Above them lie Eocene layers rich in foraminifera, tiny fossil shells that indicate the water deepened to several hundred meters. Higher still, Oligocene and Miocene mudstones and chalks show that the canyon floor was once part of a calm, upper-bathyal environment — roughly 200 to 700 meters deep. In other words, long before the canyon we see today existed, this region was already under the ocean, accumulating sediments that would later be cut open as sea levels dropped and the ancient rivers returned.

By combining seismic data, rock samples, and bathymetric mapping, scientists can even trace the influence of deep geological structures beneath the canyon. Its main axis lines up neatly with a network of ancient rift faults — fractures that date back to the breakup of Gondwana. One of these structures, called the Harvey Transfer Zone, likely guided the canyon’s course as it cut through the continental slope. Other faults formed boundaries for massive blocks of crust that tilted and rotated as the rift evolved, creating natural weak zones for erosion to exploit. This tectonic inheritance explains the canyon’s oddly sinuous shape and its sharp bends, some of which occur almost at right angles. Even millions of years after the last major tectonic movement, those ancient scars still control the landscape of the sea floor.

A Fossil Canyon Alive with Remnants of Life

It’s strange to think that a canyon of such magnitude, carved by such powerful forces, now lies in near-total silence. The Perth Canyon has effectively gone dormant. Sediment input is minimal because the nearby coastline is arid and stable; there are no large rivers feeding it anymore. Without new sediment to trigger turbidity currents, the canyon no longer experiences the dramatic flows that once sculpted it. The floor is calm, covered by fine muds that settle gently out of the water. The steep slopes, while riddled with scars of past landslides, are now static. Even seismic tremors fail to stir much movement.

Yet, even in its dormancy, the canyon is alive in another way. When the Falkor expedition lowered its remotely operated vehicle down into the depths, they didn’t just find rock and sediment — they found life, and in places, remnants of life that once flourished in astonishing abundance. Along the canyon walls, ROV cameras captured coral colonies clinging to ledges and overhangs, some living, others long dead. In the deeper sections, entire coral graveyards were discovered — fields of fossilized skeletons preserved in the fine mud. Radiometric dating showed these corals thrived between about 33,000 and 18,000 years ago, during the Last Glacial Maximum, when global sea levels were roughly 120 meters lower than today and the oceans were colder.

It’s likely that during those ice-age periods, the canyon experienced stronger currents and greater nutrient upwelling, providing food and favourable conditions for coral growth. When the climate warmed and sea levels rose, those conditions changed, and the coral ecosystems dwindled. Today, only scattered survivors remain, along with the bones of their ancient reefs.

What makes the Perth Canyon so fascinating isn’t just its size, but its dual identity. It’s both a window into Earth’s deep past and a mirror for the planet’s present. Geologically, it records the ancient dance of continents, the rise and fall of sea levels, and the shift from river-dominated erosion to the slow, steady processes of the deep ocean. In many ways, it’s the perfect symbol of Australia’s western margin: vast, old, and deceptively calm, hiding unimaginable complexity beneath the surface.

Standing on the shore near Rottnest Island, it’s easy to forget that such a world exists just beyond the horizon. But if you could drain the ocean, you’d see it — a labyrinth of cliffs, ridges, and terraces stretching westward into the abyss. You’d see the ghostly imprint of ancient rivers frozen in the rock, and the delicate remains of coral cities that bloomed and died. The Perth Canyon reminds us that even the most stable coastlines are not static. Beneath the waves, the Earth’s memory is etched in stone, waiting for us to explore it — one ROV dive at a time.

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