Mapping ties the Nullarbor's kilometre-long trenches to cave roofs falling in below

Depth kept them out of sight. The trenches are less than 9 metres deep ^[7], so 9 metres of relief spread across 500 metres of width is a cross-slope of under 2 percent ^[17]; measured along the long axis, the ratio is roughly one part in 2,000 ^[18]. Lipar said the surface features were so subtle that the trenches were hard to recognise as evidence of caves at all ^[19]. So the team stretched the vertical scale. "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 said ^[8].

A long, shallow, sediment-filled depression in limestone country looks like a valley, and shape alone would not settle the origin. Lipar said as much: "From the surface, these features can look remarkably like shallow valleys or drainage channels, but our evidence shows they have a very different origin" ^[6]. The drainage network a river would leave behind separates the two readings. "Unlike typical valleys, the trenches don't have connected streams or other signs that water once flowed through them. Instead, geophysical surveys found deep underground cavities, while several trenches lined up with known caves and areas where the ground has collapsed," he said ^[9].

In the west, collapse features connect visibly to caves; farther east the trenches are wider and subtler, and the team attributes the change to collapse propagating upward through thicker rock ^[11]. That east-west pattern carries more weight than any single trench, since one depression above one cavity is compatible with several histories. Progressive roof failure would produce a systematic change in surface expression as the overlying rock thickens, and Lipar described that sequence: "Over time, the cave roofs progressively collapsed, causing sagging of overlying material and eventually creating shallow depressions at the surface" ^[10].

By Barham's account the Nullarbor is the favourable case. "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 ^[12]. He calls the result "a useful set of indicators for identifying hidden cave systems elsewhere" ^[13], and the paper offers Mars and other planetary surfaces as places to apply them ^[15]. The identification here rested on four streams combined: mapping, geophysical surveys, cave records and sediment analysis ^[5]. Reading a Martian trough the same way would put the whole inference on topography.

On Earth the use is nearer to hand. Barham said caves "can preserve evidence of past environments on Earth and impact our engineering and access to clean drinking water, while potential caves on other planets could provide protected subsurface environments relevant to the search for evidence of extraterrestrial life or act as bases for future astronauts" ^[14]. Lipar was an adjunct research fellow at Curtin's School of Earth and Planetary Sciences during the work and is now at the Anton Melik Geographical Institute at ZRC SAZU ^[4]. The paper, "Subdued surface expression of deep cave collapse," also involved researchers from the University of Western Australia, La Trobe University, the University of Ljubljana and the University of Queensland ^[3][16].