Giant solar balloons equipped with sensitive microphones were able to pick up unexpected sounds after reaching 70,000 feet above the surface, where they entered the “stratosphere,” according to CNN.

According to the US space agency (NASA), the “stratosphere” is the second layer of the Earth’s atmosphere, and at its lowest level is the “ozone” layer, which absorbs and scatters the sun’s ultraviolet rays.

NASA explained that the thin, dry air in the stratosphere, where jet aircraft and weather balloons reach their maximum altitude, is a relatively calm layer of air that is rarely affected by turbulence.

Daniel Bowman, principal scientist at Sandia National Laboratory in New Mexico, was inspired to investigate the layer’s acoustics by studying the low-frequency sounds produced by volcanoes. This phenomenon is called ultrasound and cannot be heard by the human ear.

Bowman and his team had previously attached cameras to weather balloons to capture the black sky above and the Earth below, and then successfully built their own solar balloon.

Bowman suggested attaching infrared recorders to balloons to record volcanic sounds, but working with Jonathan Lees, a consultant at the University of North Carolina, realized that no one had attempted to put microphones in stratospheric balloons for half a century.

“Together with Lees, who specializes in Earth, ocean and environmental sciences and has experience conducting research in seismic and volcanoes, we decided to move forward to explore what this new site could do,” Bowman said.

According to Bowman, these balloons are equipped with sensors twice as fast as commercial aircraft.

“In our solar balloons, we have recorded chemical explosions, thunder, marine accidents, helicopters, city sounds, auxiliary rocket launches, earthquakes, freight trains and jets,” Bowman said in an email.

He continued, “We have recorded other sounds, but their origin is unclear.”

In a recording shared by Bowman from a NASA balloon orbiting Antarctica, the ultrasound of ocean waves sounds like a continuous sigh, but explosions and other shocks are of unknown origin.

On Thursday, during that engagement, some of the (Stratosphere) aircraft had mysterious infrared signals a few times an hour, but their source was completely unknown,” Bowman said.

Bowman and his collaborators conducted research using balloons from NASA and other aeronautical service providers, but decided to build their own. They are 6 to 7 meters in diameter.

Besides the possibility of assembling the balloons on the basketball court, he pointed out that the materials for making these balloons can be found at hardware and fireworks supply stores.

“Each balloon is made of painter’s plastic, shipping tape and charcoal dust, and costs about $50 each,” Bowman said via email.

He explained, “A two-man crew can raise a balloon in no more than three and a half hours, and if it is launched on a sunny day after being inflated, it can carry a pound of weight and reach an altitude of 70,000 feet.”

These balloons are usually coated with coal dust to darken them, and when sunlight hits them, the air inside makes them float in the air and fly away.

These inexpensive balloons, experts say, help researchers collect as much data as possible. Bowman said he launched dozens of solar balloons to collect infrared recordings from 2016 to April of this year.

Precision gauges originally designed to monitor volcanoes were attached to balloons to record low-frequency sounds.

The researchers used the Global Positioning System (GPS) to track their balloons as they traveled hundreds of miles before landing in remote locations.

The longest flight ever on NASA’s helium balloon was 44 days, and it recorded 19 days worth of data before the batteries in the microphone ran out.

Meanwhile, solar balloon flights last about 14 hours in summer and land after sunset.

The advantage of the high altitude that balloons can reach is that the noise is less, the detection range is greater, and the entire Earth can be explored.

Despite this, the balloons pose challenges for researchers because the stratosphere is a harsh environment where temperatures can fluctuate between hot and cold, according to CNN. “The solar balloons are a bit flimsy, and we’ve knocked some out of the bushes trying to get them going,” Bowman said.

And he added, “We had to walk through valleys and mountains to get our loads, and our colleagues at Oklahoma State once had to spend all night in a field to find a balloon, and then fly it again all day.”

Identifying signals recorded during flights is now the biggest challenge for researchers. No doubt it was normal or caused by wind turbulence.

For her part, Sandia National Laboratories geophysicist Sarah Albert studied the “sound wave” located at the height determined by Bowman’s research.

An acoustic pipe is a channel that transmits sounds over long distances through the atmosphere.

Sarah’s recordings have captured missile launches and other unknown sounds. “This sound can get stuck in the channel and reverberate until it’s completely jammed,” Bowman said.

And he added, “But it is not yet clear whether it is near and somewhat quiet (like a turbulent place) or far and loud (like a distant storm).”

Bowman and Sarah Albert will continue to study atmospheric sound waves to try to determine the source of the “stratospheric rumble” and why some aircraft record it while others do not.

Bowman is interested in understanding the sonic landscape of the stratosphere and unlocking key features such as variation across seasons and locations.

Helium-filled versions of these balloons could one day be used to explore other planets, such as Venus, as test flights for larger and more complex missions that would carry scientific instruments above or within the planet’s clouds for days at a time.