BioEd Online

The mystery of how the world's largest stationary sand dunes stay upright has finally been uncovered. Geologists had been baffled at how the 500-metre-high dunes in China's Badain Jaran desert remain standing in the face of savage winds.

The dunes are held together by a previously undiscovered groundwater system, report Ling Li of the University of Queensland, Australia, and colleagues. Although the dunes are dry on the outside, water below the surface sticks the grains together, allowing them to stand firm against the elements.

The researchers hit water by digging just 20 centimetres below the dunes' surface, they report in this week's Nature¹. And when they dug a one-metre-deep well in a dune's flank, water began to seep into it, despite the fact that the well was 17 metres above the level of one of the nearby lakes.

The water comes from melting snow on Qilian Mountain, 500 kilometres to the southwest of the desert, the team discovered. It percolates through cracks in the mountain and into layers of carbonate rock, eventually finding its way into the dune system.

Water result

The dunes are surrounded by 72 lakes, with a total surface area of 23 square kilometres. But nobody suspected that there would be so much extra water inside the dunes themselves - the researchers calculate that some 500 million cubic metres passes through the region each year.

"Our finding could transform plans for the region's water resources," they add. Chinese authorities are planning a large water-diversion project near Qilian Mountain, which would shift 250 million cubic metres of water a year.

That may now be unnecessary, the researchers suggest. But they warn that taking water from the dunes could cause them to move or break apart, with potentially severe effects on the local ecosystem.