Ground imaging traces the Nullarbor's mystery trenches to collapsed cave roofs

Unlike typical valleys, the trenches have no connected streams and nothing else to show that water once flowed through them ^[8]. Lipar said there is no evidence the wind cut them either ^[7]. "They look like some kind of river channel, but they start and end so abruptly that it triggered us to investigate them," said Matej Lipar of the Anton Melik Geographical Institute in Slovenia ^[3][2]. He is the study's first author.

The trenches are only 1 to 9 meters deep ^[5]. Shallow relief on a flat plain is hard to work with in the field, so the team started with digital terrain models exaggerated in the vertical. That made the depressions legible and showed where to put instruments ^[9]. Then came electrical resistivity tomography, which measures how electrical currents flow through the ground and separates rock from sediment ^[10]. The profiles put each trench at the top of a canyon-like form reaching more than 40 meters down ^[11].

Against more than 40 meters of fill, 9 meters of surface relief is about a fifth, and 1 meter is a fortieth ^[1]. "Over time, the cave roofs progressively collapsed, causing sagging of overlying material, eventually creating these shallow depressions at the surface," Lipar said ^[14].

Lipar attributes the caves themselves to groundwater draining toward the coast through porous karst limestone. The dissolution ran through the Oligocene, 33.9 million to 23 million years ago, and the Early Miocene, 23 million to 16 million years ago ^[13]. The plain has existed for more than 14 million years ^[6]. If dissolution ended with that Early Miocene interval, the roofs have had at least 16 million years to give way ^[2].

Andrea Zerboni, an Earth scientist at the University of Milan, was not involved in the work. Zerboni told Live Science 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" ^[18]. The case is a match of geometry against geometry, imaged fill lining up with catalogued caves, confirmed on the ground in the one cave the team could walk into.

Maximilian Dröllner, a geoscientist at the University of Göttingen, was also not involved. Dröllner told Live Science 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" ^[20].