Here’s what you’ll learn when you read this story:

• For more than 50 years, people around the world have reported a low-frequency hum. It was unique to coastal U.K. cities at first, but spread throughout the world.
• Theories suggest it could be factory noise, jet stream shear, or even the auditory hum of the planet itself.
• A new study analyzes the hearing of 28 people who reportedly hear “the Hum” and suggests that low-frequency tinnitus is at least partially the cause for the global phenomenon.

In the early 1970s, the town of Bristol, U.K., was plagued by a low, persistent hum. It typically occurred at night, and it irritated local residents so much that they wrote letters to the local Bristol Evening Post to complain. A variety of theories soon sprang up to explain the source of the unexplainable audio nuisance— according to the BBC, factory noise, electricity pylons, tinnitus, flying saucers (of course), and a secret government project were all put forth as possibilities.

This humming phenomenon—which eventually became known simply as “the Hum”—likely would’ve remained a peculiar footnote of history, if not for something completely unexpected happened just a few years later: The sound was heard elsewhere. Initially, it remained along the U.K. coastline in places like Hythe, Plymouth, Southampton, and Swansea. But then it migrated to London. While experts continued to struggle to explain the phenomenon, decades later, a similar low hum was heard in the U.S.—first in Taos, New Mexico and then in Kokomo, Indiana. Today, the World Hum Database Project, which has been tracking reports of the Hum across the world since 2012, includes hundreds of cases worldwide (though most appear concentrated in Europe and the United States).

Scientists haven’t found the answer in the past 50 years, but they’ve expanded the list of possible explanations. According to The Guardian, experts at an Institute of Biology Conference back in 1973 proposed that the Hum’s source was the jet stream shearing against slower moving air. In 2015, a team of French scientists theorized it could be caused by the continuous waves of the ocean. Now, in a new study published in the journal PLOS One, a team of scientists from Germany and Norway has taken a different approach to attempting to identify the source of this phenomenon—directly testing the people who hear sonic phenomena resembling the Hum, which are known scientifically as low frequency sound precepts (LFSPs).

“We know that there are people who hear low-frequency sounds that can actually be measured, even if other people don’t hear them,” Norwegian University of Science and Technology’s Markus Drexl, the senior author of the study, said in a press statement. “But it’s not so easy to find the source of these sound waves, because it’s a struggle to localize low-frequency sounds.”

Drexl and his team conducted a study that included 28 people in Germany, all of whom could hear the Hum. First, the researchers measured how well the participants could hear external noises at low frequencies, and found that only two of them had particularly good hearing in this range. While Drexl admits that sometimes people can hear within narrow bands at low frequencies, it’s unlikely that the people afflicted with this persistent Hum all naturally have good low-frequency hearing.

Drexl then analyzed whether the participants were hearing otoacoustic emissions—sounds produced by their own cochleae.

“Most of us don’t hear these sounds,” Drexl said in a press statement. “However, a few people can actually hear the sounds that the ear itself produces. And these sounds can be measured objectively.”

But again, the search came up empty. This left the researchers with only one possible option: Most of these LFSPs must be coming from low-frequency tinnitus produced by the auditory system itself.

“What we know about the hearing system is mainly based on how we capture and process sound with higher frequencies,” Drexl said in a press statement. “We know less about how the auditory system handles and processes low-frequency sound, or infrasound,” he continued.

Drexl admits that he can’t rule out that there actually is a low-frequency Hum out there. But at the very least, tinnitus appears to be playing a large role in explaining why so many people can hear this strange penomenon.