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Giant solar balloons were sent 70,000 feet up in the air to record sounds of Earth’s stratosphere — and the microphones picked up some unexpected sounds.

Daniel Bowman, principal scientist at Sandia National Laboratories in New Mexico and a Ph.D. alumnus of the University of North Carolina at Chapel Hill, was inspired in graduate school to explore the soundscape of the stratosphere after being introduced to the low-frequency sounds that are generated by volcanoes.

Bowman and his friends had previously flown cameras on weather balloons “to take pictures of the black sky above and the Earth far below” and successfully built their own solar balloon.

He proposed attaching infrasound recorders to balloons to record the sounds of volcanoes. But then he and his adviser Jonathan Lees of the University of North Carolina at Chapel Hill, “realized that no one had tried to put microphones on stratospheric balloons for half a century, so we pivoted to exploring what this new platform could do.”