'Alien' sounds have been captured at the edge of space.

The hisses and whistles heard in the recordings were captured 22 miles above Earth, by a student last year. The recording equipment was sent up by a graduate student as part of a Nasa project to record sound from the edges of the atmosphere.

The microphones picked up "infrasound" — frequencies so low that they can't be heard by human ears. But speeding up the recordings means that the sounds can be heard.

The recordings were taken from a helium balloon in August last year. The microphones dangled over New Mexico and Arizona and were one of 10 sent out by Nasa as part of the High Altitude Student Platform.

That helped get the first infrasound recordings ever taken at such altitudes, according to Daniel Bowman, the University of North Carolina student who captured the sounds.

Bowman described the sounds as "like the X-Files", and scientists aren't sure where they came from.

Current guesses include sound from a wind farm, the ocean, wind or vibrations from cables on the balloon, according to Live Science. The scientists will be sending more devices up as part of the 2015 run of the project, and hope that they can learn more then.

"I was surprised by the sheer complexity of the signal," Bowman told Live Science. "I expected to see a few little stripes."