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This Strange Hum Circled the Whole World. But Nobody Heard It.

A seismic recording device on Mayotte was the first to detect the hum, though not the first one people noticed. Mark Tingay (@CriticalStress_) processed the data to create this image.
A seismic recording device on Mayotte was the first to detect the hum, though not the first one people noticed. Mark Tingay (@CriticalStress_) processed the data to create this image. (Image credit: Mark Tingay)

There was a hum that nobody could hear. It was a seismic event, one that originated off the coast of Mayotte on Nov. 11, a tiny island in the waters between Madagascar and Mozambique.

From there, it circled the entire world, though it was unusual enough (un-earthquake-ish enough) that almost no one noticed, as Maya Wei-Haas reported for National Geographic. A few people paid attention though, and that sparked a hunt for the source of the hum that, she reported, still hasn't been resolved.

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The hum, National Geographic reported, was strange for a number of reasons. First, it rang at just a single ultra-low frequency, like a well-tuned bell. Seismic waves usually involve lots of different frequencies. Second, the wave emerged and circled the planet without the usual signs of an earthquake; no one in the area felt any shaking, and the "p-waves" and "s-waves" associated with the hum, the sort of waves that you actually feel during an earthquake, were so faint as to be nearly undetectable. And yet, a Nov. 12 report from the French government found that Mayotte had slid 2.4 inches (6 centimeters) to the east and 1.2 inches (3 cm) to the south. [The 10 Biggest Earthquakes in History]

Scientists have proposed a number of possible explanations for the strange seismic event near Mayotte, Wei-Haas reported. But none is yet anywhere near confirmed. Perhaps a "slow earthquake" struck the area, the sort that doesn't cause much intense shaking because it occurs over a much longer period of time. Perhaps a bubble of magma squeezed past below the surface, or sloshed around in a big hole in the crust in a way that interacted with the local geology to produce the resonant ringing. Researchers even speculated about a meteor strike, though that seems unlikely. For now, the exact cause remains a mystery.

For more on the unusual seismic event, read the full report at National Geographic

Originally published on Live Science.

Rafi Letzter
Rafi joined Live Science in 2017. He has a bachelor's degree in journalism from Northwestern University’s Medill School of journalism. You can find his past science reporting at Inverse, Business Insider and Popular Science, and his past photojournalism on the Flash90 wire service and in the pages of The Courier Post of southern New Jersey.