Ancient Cave Collapse Formed Nullarbor Plain Trenches

Five massive trenches along the eastern edge of Australia's Nullarbor Plain were formed by the collapse of ancient underground caves. This finding resolves a long-standing mystery about the origin of these stark landscape features.

Researchers used digital terrain modeling and electrical resistivity tomography to probe the structures. The techniques revealed that the surface trenches are merely the tops of sediment-filled canyons extending more than 130 feet below ground. The trenches align with known, massive cave systems accessible via sinkholes.

The Nullarbor's Mysterious Scars

The Nullarbor Plain is a flat, arid expanse that has existed for over 14 million years. Its eastern edge is marked by five long, linear depressions. These features are substantial but surprisingly subtle from ground level.

Matej Lipar, a researcher at the Anton Melik Geographical Institute in Slovenia and the study's first author, explained their puzzling appearance. "They look like some kind of river channel, but they start and end so abruptly that it triggered us to investigate them," he told Live Science. Wind or water erosion were ruled out as causes due to a lack of typical associated features like connected stream networks.

Mapping the Subsurface Void

The investigation combined several methods. First, the team ran exaggerated digital models to make the subtle trenches visible and plan their fieldwork. They then employed electrical resistivity tomography, which measures how electrical currents move through the ground to distinguish solid rock from loose sediment.

The data painted a clear picture. Each surface trench sat atop a deep, sediment-filled chasm. The sediment itself was layered, with sand on top and finer silt and clay below. Critically, the team found that databases of known caves showed large systems precisely aligned beneath these trenches.

A Timeline of Collapse

The research points to a specific geological sequence. The aligned caves were formed in porous limestone by groundwater flowing toward the coast during the Oligocene and Early Miocene epochs, between approximately 33.9 million and 16 million years ago.

Over immense timescales, the roofs of these giant caves progressively failed. "Over time, the cave roofs progressively collapsed, causing sagging of overlying material, eventually creating these shallow depressions at the surface," Lipar said. The team confirmed the model by entering Clay Dam Cave, accessible via a sinkhole within one trench, and finding its sagging layers and sediment infill matched their predictions.

Independent experts praised the work. Andrea Zerboni, an Earth scientist at the University of Milan not involved in the study, told Live Science 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.

Implications Beyond Earth

The study suggests the process is ongoing, as some trenches appear fragmented, indicating recent or future collapses. It also has implications for planetary science. Similar elongated depressions on Mars are often interpreted as collapsed lava tubes.

Zerboni noted this similar surface features could also result from karst-like processes. This insight could guide the search for extraterrestrial life. Subsurface cavities are promising environments where past life, or traces of it, might be shielded from harsh surface radiation and weathering.

Geoscientist Maximilian Dröllner of the University of Göttingen added that the methodology could help identify similar caves elsewhere and be valuable for studying potentially habitable environments on other planetary bodies. The work ultimately transforms the understanding of a stark Australian landscape, revealing the hidden voids that shaped it.