China’s Badain Jaran desert, which covers more than 40,000 square kilometers, is home to some of the world’s tallest stationary dunes, seen here from the International Space Station. They can stand 500 meters tall and cast immense shadows. What keeps these piles of sand immobile despite arid, windy conditions is a previously unknown source of groundwater, says Ling Li, an environmental engineer at the University of Queensland in St.