Scientists who attached microphones to solar balloons found something bewildering in the stratosphere—strange, unexplained sounds. Daniel Bowman, principal scientist at Sandia National Laboratories in New Mexico, shared findings from the eerie recordings—made about 13 miles above Earth’s surface—at a meeting of the Acoustical Society of America on Thursday. The low frequency noises are infrasound waves, meaning they are imperceptible to the human ear but can be picked up with special microphones. “On our solar balloons, we have recorded surface and buried chemical explosions, thunder, ocean waves colliding, propeller aircraft, city sounds, suborbital rocket launches, earthquakes, and maybe even freight trains and jet aircraft,” Bowman said. “We’ve also recorded sounds whose origin is unclear.” He added that in the stratosphere, “there are mysterious infrasound signals that occur a few times per hour on some flights, but the source of these is completely unknown.” The inexplicable rustling and crackling sounds could be caused by a natural phenomenon like a distant storm or something human-made like a train, Bowman said, “but it’s hard to tell what is going on sometimes due to the lack of data up there.”