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.”