Immense, almost invisible trenches on the surface of Australia reveal the presence of deep, hidden caves.
These formations cross the Nullarbor Plain in the south of the country. Some extend for more than 20 km and are generally between 100 and 500 m wide. Yet their depth remains less than 9 m.
View from inside the Murrawijinie Caves on the Nullarbor Plain, Australia.
Credit: Stephen Mabbs / Unsplash.
At first glance, these elongated depressions resemble ancient valleys carved by water. However, researchers found that they lack the expected drainage network. The sediments also show none of the characteristic traces of a former organized flow.
To understand their origin, the team combined several methods. It studied the shape of the terrain, known caves, sediments and cliffs where the rock layers are visible. Geophysical measurements also made it possible to indirectly probe what lies beneath the surface.
The result links these trenches to much deeper cavities. In the region, some caves descend between 50 and 150 m below the ground. When their ceilings gradually collapse, the rocks above them sink in turn. This movement can slowly reach the surface and form a long depression there.
Collapse sinkholes and rock deformations associated with the trenches studied on the Nullarbor Plain.
Source: Lipar et al., Communications Earth & Environment, CC BY-NC-ND 4.0.
In other words, this is not necessarily a major, sudden collapse that opens up a chasm. The ground may gradually sink above an underground passage. Over time, sediments partly fill the weakened area. The final landform then becomes very subtle despite the size of the structure beneath it.
The Nullarbor Plain provides favorable conditions for preserving these traces. This limestone platform, covering more than 200,000 km², has been exposed for more than 14 million years. Its dry climate greatly limits the action of rivers, which could otherwise alter or erase these subtle landforms.
This discovery also provides clues for searching for cavities elsewhere without directly seeing their entrances. A long, shallow depression with no genuine drainage and associated with other signs of collapse may indicate an underground network. This way of interpreting the landscape could be useful in geology, as well as in certain development projects or the study of groundwater.
Finally, the researchers are considering applying these criteria to other worlds. On Mars, some elongated depressions have been attributed to ancient flows. In some cases, an underground cavity followed by its collapse could offer another explanation. Planetary caves are of particular interest to scientists because they can protect their interiors from the harsh conditions at the surface.