The balloons can take sensors twice as high as commercial jets can fly.
Stratosphere, a layer of atmosphere above the Earth, is believed to be calm and quiet. But when scientists sent a solar-powered balloon in the zone between about 10 and 50 kilometres above the surface of the earth, they detected mysterious noises with unknown origin. According to Washington Post, the noises are "infrasound" - inaudible to the human ear, like infrared light is not visible to the human eye. The outlet quoted scientists to say that when recorded with specialised instruments and sped up a few thousand times, it sounded like muffled whispers.
Scientists separated these mysterious noises from other sounds captured from the zone, like the distant sound of ocean waves slamming into each other.
"We spend an inordinate amount of time arguing about what things are," Siddharth Krishnamoorthy, a research technologist at NASA's Jet Propulsion Laboratory in California's Pasadena, told the Post.
"I've been doing it for about 10 years now, and, you know, the fact that there's mysterious sounds that I don't understand is troubling, but it's not like a revelation," said another researcher Daniel Bowman.
Mr Bowman and his friends have previously sent balloons up in the air to take pictures of the black sky and Earth below. They have also successfully constructed their own solar-powered balloons, said CNN.
The experiment was carried out after Mr Bowman "realised that no one had tried to put microphones on stratospheric balloons for half a century", according to the outlet.
The balloons can take sensors twice as high as commercial jets can fly.
Mr Bowman and his team presented the findings on Thursday at the 184th Meeting of the Acoustical Society of America in Chicago.