Giant caves beneath sinkhole reveal origin of mystery trenches that score Australia's Nullarbor Plain

Mysterious trenches stretching for miles across the vast Nullarbor Plain in Australia are the surface scars of huge ancient caves collapsing deep underground, according to new research.

"They look like some kind of river channel, but they start and end so abruptly that it triggered us to investigate them," Matej Lipar, a researcher at the Anton Melik Geographical Institute in Slovenia and first author of the study, told Live Science.

The Nullarbor Plain is an arid, flat, almost-treeless expanse in southern Australia. Nestled between the Great Victoria Desert and the Great Australian Bight coast, it has existed for more than 14 million years. Along the eastern edge of the Nullarbor Plain are five long trenches with similar north-to-south orientations. They range from several miles to more than 12 miles (20 kilometers) long, span 330 to 1,640 feet (100 to 500 meters) wide and extend between 3 and 30 feet (1 to 9m) deep.

Despite their large size, the trenches aren't always easy to spot. "You don't really notice them when you are on the Nullarboor because they are really flat," Lipar said.

How the trenches formed wasn't clear. There is no evidence that they were created by the wind, Lipar noted. And it's.unlikely that they were carved out by flowing water, because unlike typical valleys, the trenches don't have connected streams or other signs that water once flowed through them, Lipar explained.

To investigate the mysterious trenches' origin, Lipar and his colleagues ran exaggerated digital models of the terrain to make the trenches easier to see and identify where the team needed to focus. Then, they used an imaging technique called electrical resistivity tomography ‪—‬ which measures how electrical currents flow through the ground ‪—‬ to distinguish rock from sediment.

The results revealed that the trenches were dips at the top of what seemed almost like canyons stretching down more than 130 feet (40 m) below the surface. They were filled with sediment, with sand dominating the upper layers and higher proportions of fine particles of silt and clay below that.

When the team looked at a database of known caves, they found that some big, deep caves were lined up along the trenches. The accessible ones can often be reached via sinkholes — also known as dolines — and contain massive passages that can stretch for hundreds of metres. Their similar orientation to the trenches suggests the caves were formed in the porous, karst limestone area by groundwater flowing toward the coast during the Oligocene (33.9 million to 23 million years ago) and Early Miocene (23 million to 16 million years ago), Lipar said.

"Over time, the cave roofs progressively collapsed, causing sagging of overlying material, eventually creating these shallow depressions at the surface," Lipar said. The work was published Sept. 17 in the journal Communications Earth and Environment.

"What this also tells you is that when the trenches end, we might still have a cave that hasn't collapsed yet," Lipar said, and that might help us locate big caves that have never been seen by human eyes before.

The team confirmed their ideas by entering Clay Dam Cave, which is accessible by a sinkhole within a trench, finding that its sagging strata and sandy sediment infill matched what they'd detected in the other trenches.

"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," Andrea Zerboni, an Earth scientist at the University of Milan who wasn't involved in the study, told Live Science.

The fact that some trenches are fragmented implies this process is ongoing, and there might be more collapses in the future, Lipar said.

Maximilian Dröllner, a geoscientist at the University of Göttingen in Germany who wasn't involved in the study, told Live Science 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."

Similar features have been seen in other deserts, including parts of the Arabian Peninsula, where networks of collapsed caves can be recognized at the surface as elongated depressions, Zerboni noted.

And beyond Earth the implications could be particularly intriguing. "Elongated depressions on Mars have often been interpreted as collapsed lava tubes," Zerboni said. "However, this study shows that superficially similar structures may also result from the collapse of underground systems formed by karst-like rock-alteration processes."

This insight might help scientists pinpoint good places to look for signs of extraterrestrial life. "Subsurface cavities are among the most promising environments in which past life — or traces of it — might have been protected from radiation, extreme temperatures and surface weathering," Zerboni said.