Ever wonder what near space sounds like? The mystery is solved thanks to a NASA balloon experiment that captured atmospheric infrasound 22 miles above Earth. Infrasound, at frequencies below 20 hertz, is undetected to the human ear, but becomes audible when sound waves are sped up. Sensors on a helium balloon let loose above New Mexico and Arizona in August managed to record hours of audio, including stratospheric signals researchers say they've never heard before, at altitudes of up to 123,000 feet, never reached by such an experiment. The noise, which you can listen to here, sounds like various whistles and hisses. Or as grad student Daniel Bowman of the University of North Carolina tells LiveScience: "It sounds kind of like The X-Files."
"There haven't been acoustic recordings in the stratosphere for 50 years," Bowman, the experiment lead, says. "Surely, if we place instruments up there, we will find things we haven't seen before." Experts say the noises likely came from several sources: ocean waves, wind turbulence, gravity waves, air turbulence, vibrations from the balloon cable, and a wind farm the balloon flew over. A second balloon experiment this year could offer further insight and capture more intriguing sounds. LiveScience notes interest in atmospheric infrasound as a means of detecting nuclear explosions was high in the 1960s, before ground-based sensors became commonplace. However, scientists have more recently explored the idea of sending infrasound sensors to Venus and Mars to monitor weather or earthquakes. (Thanks to NASA, Mercury has a new crater.)