The study, published in Communications Earth & Environment under the title ‘Subdued surface expression of deep cave collapse’, combined detailed mapping, geophysical surveys, existing cave records and sediment analysis to build a case that is as much about method as it is about discovery. Lead author Dr Matej Lipar, who conducted the work as an Adjunct Research Fellow in Curtin’s School of Earth and Planetary Sciences and is now at the Anton Melik Geographical Institute at the Research Centre of the Slovenian Academy of Sciences and Arts (ZRC SAZU), explained that the features are deceptively ordinary in appearance. From the surface, he noted, they can look remarkably like shallow valleys or drainage channels, yet the evidence shows they have a fundamentally different origin. Instead of being cut from above by running water, they are the end product of sagging ground above caverns dissolved deep within the limestone.

The scale of the features is striking. The trenches range from several kilometres to more than 20 kilometres in length and are generally between 100 and 500 metres wide. Yet their depth is modest: less than nine metres. On a plain as immense and flat as the Nullarbor, that vertical expression is nearly invisible to anyone standing on it, and even conventional imagery struggles to reveal the pattern. The researchers found that exaggerated digital models of the terrain, in which vertical relief is artificially amplified, made the trenches dramatically easier to see and identify. This technique of terrain exaggeration, long used by geomorphologists to tease out faint topographic signals, proved decisive in revealing the full extent of a phenomenon that had been hiding in plain sight across one of the world’s largest karst regions.

What convinced the team that water was not the sculptor was the absence of the usual signatures of fluvial erosion. Typical valleys on Earth, even dry ones, carry the imprint of the streams that formed them: connected drainage networks, channels that converge downstream, sediment deposits sorted by flowing water, and terraces left behind as flows waxed and waned. The Nullarbor trenches show none of this. As Dr Lipar observed, the trenches lack connected streams or any other indication that water once moved through them. In their place, geophysical surveys detected deep underground cavities beneath the surface depressions, and several of the trenches aligned directly with known caves and documented areas of ground collapse. The correlation between surface expression and subsurface voids provided the physical link the team needed to reinterpret the trenches as collapse features rather than channels.

The mechanism the researchers describe is one of slow, progressive failure. In karst landscapes, slightly acidic groundwater dissolves limestone along fractures and bedding planes, opening caverns that can grow to enormous dimensions over millions of years. The Nullarbor Plain, a former seabed of Miocene-age limestone lifted clear of the ocean and exposed to the air, is one of the classic karst terrains of the world, honeycombed with caves that in places extend far below the surface. When a cave roof is thick and strong, the cavern can persist indefinitely. But over geological time, roofs weaken. Support pillars thin, fractures propagate, and chambers begin to fail from the top down. Each small collapse deposits rubble on the cave floor and transfers stress upward through the rock column. Eventually, the cumulative sagging of the overlying material propagates all the way to the land surface, producing a shallow, broad depression rather than a dramatic sinkhole.

That progressive character is written into the geography of the trenches themselves. The team identified a clear spatial progression: in the west of the study area, the features are obvious cave-connected collapse structures, where the rock cover above the caverns is thinner and failure has broken through more completely. Moving eastward, the trenches become wider and more subtle, reflecting caverns buried beneath thicker sequences of rock where collapse has deformed the surface gently rather than rupturing it. This west-to-east gradient, the researchers argue, demonstrates how cave collapse can reach the surface at different stages depending on the thickness and strength of the intervening rock. It effectively turns the Nullarbor into a natural laboratory in which the full life cycle of cave collapse, from intact cavern to subdued surface trench, can be read across the landscape.

Co-author Associate Professor Milo Barham, from the Curtin Frontier Institute for Geoscience Solutions and the School of Earth and Planetary Sciences, emphasised that the Nullarbor’s peculiar qualities make it an ideal place to learn this lesson. Cave systems, he noted, are not always obvious from the surface, but the dry and stable landscape of the Nullarbor strips away much of the noise that obscures subtle landscape characteristics elsewhere. In humid, vegetated, actively eroding terrain, collapse depressions are quickly modified by soil creep, vegetation, slope processes and fluvial overprinting, making their origin difficult to disentangle. On the Nullarbor, where erosion rates are extraordinarily low and the surface has been preserved with remarkable fidelity, the faint signatures of deep collapse survive intact, offering a clean set of indicators that can be applied to far messier landscapes.

The implications extend well beyond southern Australia. Barham pointed out that the findings provide a useful set of indicators for identifying hidden cave systems elsewhere, including on other planets such as Mars. Planetary scientists have long known that Mars hosts candidate caves: collapse pits, skylights and elongated depressions traceable to lava tubes and possible groundwater-carved caverns, imaged from orbit by spacecraft instrumentation. Because orbital data are often the only evidence available, and because the resolution of that data can make subtle relief hard to interpret, recognising subdued surface expressions of deep collapse could substantially expand the inventory of identifiable subsurface voids on the Red Planet. The Nullarbor, with its aridity, stability and faint but legible collapse features, serves as a terrestrial analogue for exactly the kind of terrain where such signatures might be sought on Mars and other planetary surfaces.

The stakes of that search are considerable. Caves on Earth are far more than geological curiosities. They preserve evidence of past environments, from fossil-bearing sediments to mineral deposits and climate archives locked in speleothems, making them invaluable records of how landscapes and climates have changed over millions of years. They also matter in practical terms: Barham noted that caves influence engineering decisions and access to clean drinking water, since karst aquifers supply water to large populations and their unpredictable voids pose hazards for construction and infrastructure. On other planets, potential caves could provide protected subsurface environments relevant to the search for evidence of extraterrestrial life, shielding any biosignatures from radiation and harsh surface conditions, or act as natural shelters and bases for future astronauts. Knowing where to look for them, and how to read their faint surface traces, is therefore a prerequisite for much of that future work.

The study was supported by the Slovenian Research and Innovation Agency and involved researchers from The University of Western Australia, La Trobe University, the University of Ljubljana and The University of Queensland, alongside the Curtin team. Its broader message is a reminder of how much can remain hidden beneath even the most familiar-looking ground. The Nullarbor appears, as Dr Lipar described it, to be a flat, endless plain, yet exaggerated digital terrain models and geophysical probing reveal kilometres-long scars of a hidden underworld slowly caving in. For geomorphologists, the work adds a new diagnostic framework: elongated, shallow, drainage-free trenches in dry karst terrain should no longer be assumed to be valleys, but read instead as the quiet surface record of deep caves in the long process of collapse. For planetary science, it offers a template for finding sheltered subsurface spaces on worlds where no explorer has yet set foot.

Subject of Research: Surface expression of deep cave collapse in the karst landscape of the Nullarbor Plain

Article Title: Hidden caves beneath the Nullarbor revealed by subtle surface

Article References: Hidden caves beneath the Nullarbor revealed by subtle surface. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Nullarbor Plain, karst, cave collapse, geomorphology, digital terrain modelling, geophysical surveys, limestone caves, Curtin University, Mars analogue, sinkholes, Communications Earth & Environment, planetary science