Solar-powered balloons floating in the stratosphere have recorded low-frequency sounds of mysterious origin. "When we started flying balloons years ago, we didn't really know what we'd hear," says Daniel Bowman at Sandia National Laboratories in New Mexico. "We learned how to identify sounds from explosions, meteor crashes, aircraft, thunderstorms and cities. But virtually every time we send balloons up, we find sounds that we cannot identify." Bowman and his colleagues measured infrasound signals – sounds with a frequency so low they are inaudible to human ears – using solar-powered balloons floating 20 kilometres high.