Mysterious, sediment-filled trenches stretching up to 20 kilometres across southern Australia’s vast Nullarbor Plain are the surface scars of ancient caves collapsing deep underground, according to a study published on September 17 in Communications Earth and Environment. The discovery resolves a geological puzzle in the arid limestone landscape.
For millions of years, the flat, arid expanse of southern Australia’s Nullarbor Plain has hidden a remarkable underground world. Stretching over 200,000 square kilometres across limestone formations that emerged when an ancient shallow sea retreated roughly 14 million years ago, the landscape has long acted as a significant barrier. Colonial Victorian-era explorers described it as the sort of place one gets into in bad dreams
, while the Mirning People have lived along its southern margin for tens of thousands of years, watching sea levels rise and fall.
To an untrained eye, the region appears deceptively simple and featureless. But scattered across the surface are long, shallow trenches trending north-to-south that span 100 to 500 metres wide and reach lengths of up to 20 kilometres, while staying generally under nine metres deep.
Investigating Australia’s Hidden Linear Trenches
Because the terrain is exceptionally flat, these features are nearly impossible to notice from the ground. Researchers realized they needed a different perspective to understand them, as the trenches are difficult to recognise as evidence of underground caves because their surface features were so subtle.
To solve the mystery, scientists combined drone and aerial photography, borehole measurements, cave records, and sediment analysis. When the team ran exaggerated digital models of the terrain, the mysterious structures became immediately apparent.
Despite initial appearances resembling flat-bottomed valleys, investigators found no connected streams, drainage networks, or sediment deposits that would indicate flowing water ever carved them. Instead, specialised geophysical techniques like electrical resistivity tomography—which measures how electrical currents flow through the ground—revealed that the trenches sit directly above deep zones of broken rock and massive underground cavities.
How Ancient Karst Processes Formed the Nullarbor Depressions
The subterranean voids beneath the plain formed during the Oligocene and Early Miocene epochs, between 33.9 million and 16 million years ago, as groundwater flowed toward the coast through porous limestone. Over immense spans of time, the roofs of these giant caverns gave way.
The research team confirmed this mechanism by entering Clay Dam Cave, a system accessible via a sinkhole located within one of the trenches. Inside, they observed sagging strata and sandy sediment infill that matched the data collected across other trenches. The upper layers are dominated by sand, while deeper portions contain higher proportions of fine silt and clay particles.
The study also revealed a distinct geographical progression. Researchers identified obvious cave-connected collapse features in the west transitioning into wider, more subtle trenches further east, illustrating how cave collapse progressively reaches the surface depending on the thickness of the overlying rock layers.
Global Geological Implications and Predicting Unseen Caves
The discovery changes how geologists interpret similar pockmarked karst landscapes, which cover 15 percent of the ice-free Earth’s surface and supply drinking water to 10 to 25 percent of the global population. Understanding these hidden structural voids carries direct consequences for planning surface and subsurface engineering works, managing geological hazards, and protecting groundwater resources.
Furthermore, the surface expressions offer a predictive tool for speleologists. Because the trenches mark the upper limits of subterranean cavities, what this also tells you is that when the trenches end, we might still have a cave that hasn’t collapsed yet, providing a way to locate massive cave systems that remain entirely hidden from human eyes.
External experts weighed in on the broader relevance of the findings. Andrea Zerboni, an Earth scientist at the University of Milan who was not involved with the project, noted that the authors skillfully show that the linear depressions along the margin of the Nullarbor Plain are probably the surface expression of the progressive collapse of ancient underground cavities, adding that similar networks of collapsed caves appear in other deserts, including parts of the Arabian Peninsula.
Prospects for Planetary Science and the Search for Extraterrestrial Life
Beyond terrestrial applications, the findings open new possibilities for interpreting planetary landscapes where water once flowed or altered rock structures. The stable, dry landscape of the Nullarbor provides an ideal baseline because it lacks the disruptive erosion noise found in wetter environments.
Caves are important as they can preserve evidence of past environments on Earth and impact our engineering and access to clean drinking water, while potential caves on other planets could provide protected subsurface environments relevant to the search for evidence of extraterrestrial life or act as bases for future astronauts, according to Associate Professor Milo Barham of the Curtin Frontier Institute for Geoscience Solutions.
Maximilian Dröllner, a geoscientist at the University of Göttingen in Germany who was not involved in the research, noted that the work could help identify similar caves elsewhere and may be particularly valuable for the study of caves and potentially habitable environments on other planetary bodies. While elongated depressions on Mars have frequently been interpreted as collapsed lava tubes, this study demonstrates that superficially similar planetary features can also stem from karst-like rock-alteration and collapse processes.