Mysterious trenches stretching for kilometers across southern Australia's Nullarbor Plain have been identified as surface signs of deep, ancient cave systems hidden beneath the ground. Recent research involving Curtin University has revealed that these shallow, sediment-filled trenches formed when underground caves gradually collapsed, eventually reaching the surface. This challenges the common belief that such features were carved by flowing water, even though their appearance might suggest otherwise. The study, titled "Subdued surface expression of deep cave collapse," was published in the journal Communications Earth and Environment. Lead author Dr. Matej Lipar, who was an adjunct research fellow at Curtin University's School of Earth and Planetary Sciences at the time of the research and is now based at the Anton Melik Geographical Institute in Slovenia, noted that the trenches are difficult to recognize as evidence of underground caves because their surface features are so subtle. "From the surface, these features can look like shallow valleys or drainage channels, but our evidence shows they have a very different origin," Lipar said. By using mapping, geophysical surveys, cave records, and sediment analysis, the team found that the trenches are linked to deep cave systems beneath the Nullarbor Plain. The trenches can be several kilometers to more than 20 kilometers (12 miles) long and are generally 100–500 meters (330–1,640 feet) wide, but they are less than 9 meters (30 feet) deep, making them hard to spot on the vast Nullarbor Plain. "Because the landscape is so vast, it appears to be a flat, endless plain. But when we created exaggerated digital models of the terrain, the trenches became much easier to see and identify," Lipar explained. Unlike typical valleys, these trenches do not have connected streams or other signs of water flow. Instead, geophysical surveys found deep underground cavities, and several trenches aligned with known caves and areas where the ground had collapsed. The researchers also observed a pattern: from the west, where cave-connected collapse features are more obvious, to the east, where the trenches are wider and more subtle. This shows how cave collapse can gradually reach the surface, depending on the thickness of the rock above. Curtin co-author Associate Professor Milo Barham, from the Curtin Frontier Institute for Geoscience Solutions and the School of Earth and Planetary Sciences, said the findings could change how scientists interpret similar subtle landscape features elsewhere. "Cave systems are not always obvious from the surface, but the dry and stable landscape of the Nullarbor is excellent for removing a lot of 'noise' to allow us to recognize subtle landscape characteristics," Barham said. The study involved researchers from The University of Western Australia, La Trobe University, the University of Ljubljana, and The University of Queensland. The findings provide a new method for identifying hidden cave systems in dry landscapes on Earth and could help scientists interpret similar features on Mars and other planetary surfaces.