On November 11, the entire planet shook for roughly 20 minutes and yet not a single human being felt it. Though we know ‘it’ happened, we still have no idea what ‘it’ was.

The seismic event began at approximately 9:30 UT, roughly 15 miles off Mayotte, a French island located near Madagascar off Africa’s southeast coast.

The seismic waves from this event travelled as far as Chile, New Zealand and Canada, and eventually reached the US state of Hawaii some 11,000 miles away. Earthquake enthusiast @matarikipax posted images of the anomalous activity to Twitter.

This is a most odd and unusual seismic signal.Recorded at Kilima Mbogo, Kenya ...#earthquakehttps://t.co/GIHQWSXShdpic.twitter.com/FTSpNVTJ9B — ******* Pax (@matarikipax) November 11, 2018

“I don't think I've seen anything like it,” Columbia University seismologist Göran Ekström told National Geographic, before cautioning: “It doesn't mean that, in the end, the cause of them is that exotic.”

Normal earthquakes are short-lived affairs that produce a sudden jolt that typically subsides in seconds, followed by aftershocks which can last far longer. Throughout these standard seismic shifts, three types of waves are produced.

An earthquake will usually first trigger fast-travelling signals known as Primary or P-waves followed by relatively high frequency secondary or S-waves.

Thirdly, long-period surface waves travel around the globe multiple times following a big enough earthquake. While this type of wave most closely resembles the Mayotte mystery signal, there was no earthquake recorded that could have triggered such a bizarre low frequency in the way observed.

“A sort of ping rather than a rumbling,” explains Stephen Hicks, a seismologist at the University of Southampton.

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However, the Mayotte event triggered a surprisingly monotonous and low-frequency signal which further mystifies scientists; the wave was monochromatic and took roughly 17 seconds to repeat – completely different to the 'noisy' waves emitted by standard earthquakes.

SBV, like the other stations, shows long monochromatic signal with ~17s period (mono-freq Rayleigh waves?). But filtered above 1Hz SBV (lower plot) also shows seismic(?) signals from repeating sources, with some ~50s apart. Maybe some large, shallow, oscillating volcanic source? pic.twitter.com/bPqdQFwAgm — Anthony Lomax 🌍🇪🇺 (@ALomaxNet) November 11, 2018

Most theories point to a seismic swarm that has rocked Mayotte since last May, with hundreds of quakes registered from an area roughly 31 miles offshore, east of the source of the odd ringing. However, no quakes have been recorded recently and the frequency of seismic activity has subsided in recent months, further confounding scientists investigating this mysterious phenomenon.

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The current working theory is that there is a magma body, roughly a cubic mile in size, that is shunting its way through the subsurface of Mayotte and may have caused a subaquatic magma chamber collapse. Additional ideas suggest that what is known as a 'slow earthquake,' which is quieter than its traditional namesake and can last minutes, hours or even days, is what is behind the enigmatic signal.

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