Mysterious crackles, hisses and whistles have been heard 36 kilometers (22 miles) above the Earth’s surface. Unsure of their exact origin, NASA plans to send more microphones up into this region of near space to continue eavesdropping.
“It sounds kind of like 'The X-Files,” Daniel Bowman, who built the equipment that made the recordings, told Live Science. Known as atmospheric infrasound, the noises are at too low a frequency to be heard by the human ear, generally dipping below 20 hertz. Sped up, however, and the eerie sounds are revealed. Listen to the recording below.
Bowman, a graduate student from the University of North Carolina, hopes that these recordings might encourage more people to start listening to atmospheric infrasound again. The last experiments that recorded these waves from such heights occurred in the 1960s, when scientists were trying to listen out for nuclear explosions. Since then though, they’ve largely been forgotten about. “There haven't been acoustic recordings in the stratosphere for 50 years. Surely, if we place instruments up there, we will find things we haven't seen before,” says Bowman.
The infrasound microphones he built were attached to NASA’s High Altitude Student Platform (HASP). The large helium balloon is an annual project run by NASA in which university students can design experiments to be sent up into near space—the region of the atmosphere above the altitude at which planes fly, but below the limit of the stratosphere at 100 kilometers (62 miles) up.
The flight occurred last year and lasted a total of nine hours as it drifted over New Mexico and Arizona, reaching a maximum height of 37.5 kilometers (23.5 miles). This is the highest any infrasound experiment has reached, according to Bowman. In fact, the recordings were so intriguing that NASA is planning to send more recording devices up there on this year's HASP flight.
Because of their low frequency, the infrasounds carry for long distances. They can be produced by all sorts of natural events, such as volcanoes, earthquakes or thunderstorms. It’s thought that listening to these waves might even be a way of monitoring the weather. Bowman has similar plans, as he’s looking to record the infrasound above an erupting volcano.
Rather than aliens or robots though, the source of these spectral sounds are probably closer to home. Best guesses so far range from signals from wind farms, ocean waves crashing, wind turbulence, or even just the vibrations caused by the balloon cable. Looks like we won’t be needing Mulder and Scully after all.