Solar-powered balloons launched into the Earth’s stratosphere have recorded a series of mysterious rumblings, and scientists can’t pinpoint their origins.

The noises, detected by specialized instruments at 70,000 feet above the Earth’s surface, are known as infrasound because they are so low-pitched they are inaudible to human ears. Picked out from among a wash of hidden low-frequency sounds — including thunder, ocean waves, rocket launches, cities, wind turbines and even planes, trains and automobiles — the strange infrasounds have so far defied explanation.

“[In the stratosphere,] there are mysterious infrasound signals that occur a few times per hour on some flights, but the source of these is completely unknown,” lead investigator Daniel Bowman(opens in new tab), a senior scientist at Sandia National Laboratories in New Mexico, said in a statement(opens in new tab).

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