Eerie mysterious sounds were recorded by the microphones of giant solar balloons that were sent for research purposes to a height of 21 kilometers in the stratosphere.

The stratosphere is the second layer of Earth’s atmosphere, and its lowest level includes the ozone layer, which absorbs and scatters the sun’s ultraviolet radiation, according to NASA.

Daniel Bowman, principal scientist at Sandia National Laboratories in New Mexico, had aspired since graduate school to explore the soundscape of the stratosphere when he came across the low-frequency sounds produced by volcanoes. These are the so-called infrasounds, the deafening sounds produced by tsunamis, storms, earthquakes, volcanoes, avalanches that are not perceived by the human ear.

Bowman and his friends first attached cameras to weather balloons to take “pictures of the dark sky and Earth,” so they developed their solar balloon.

Bowman suggested installing infrasound recorders to record the “howls” of volcanoes. He and his advisor, Jonathan Lilas of the University of North Carolina, realized that “no one had tried to put microphones in stratospheric balloons for half a century,” so they decided to explore the possibilities of these new sensors that can fly in double of commercial aircraft altitude.

“In our solar balloons, we have recorded surface and subsurface chemical explosions, lightning, crashing ocean waves, propeller planes, city sounds, suborbital rocket launches, earthquakes, possibly commercial trains and airplanes. We have also recorded sounds of undetermined origin,” notes Bowman.

The scientists’ findings were described at the 184th Conference of the Acoustical Society of America in Chicago.

A recording shared by Bowman from a NASA balloon orbiting Antarctica contains echoes of crashing ocean waves (sounds that sound like continuous breathing). But other sounds, such as clicks and rustles, have baffled scientists as they cannot pinpoint their source.

According to Bowman, between 2016 and last month, his team launched dozens of solar balloons (equipped with microbarometers) to collect infrasound recordings.