This article originally appeared on Motherboard.
Mysterious seismic waves that cascaded around the world this month have scientists scratching their heads.
On the morning of November 11, peculiar seismic ripples were detected 15 miles off the coast of Mayotte, a French island between Mozambique and Madagascar in the Indian Ocean.
The strange waves pulsed for a total of 20 minutes, unfelt by humans, according to National Geographic on Thursday. They have not been reported to have caused any damage.
It seems the event was first noticed by Twitter user @matarikipax who shared a US Geological Survey seismogram display from Kilima Mbogo, Kenya, noting, “This is a most odd and unusual seismic signal.”
Much like the waves themselves, news of them ricocheted across Twitter to researchers speculating about their source. Theories and observations ranged from submarine volcanic eruptions and tectonic earthquakes to meteors and space weather.
“The event is almost certainly volcanic-related, since Mayotte and the region around are volcanic,” Anthony Lomax, an independent seismologist, theorized to Motherboard.
“The seismic waves may be from earthquake-like, faulting rock movement responding to inflation/deflation or collapse of a volcanic edifice, or directly related to movement or vibration of magma,” Lomax added.
Typical earthquakes trigger two kinds of fast-moving body waves at high frequencies. The first are primary or P-waves, which can travel longitudinally through rock and liquid, and are sometimes felt by humans. Secondary or S-waves can also be felt, but cannot travel through liquid, and tend to shake the ground side-to-side along its course.
These waves are rounded out by lower frequency surface waves, which can be responsible for the worst surface destruction during an earthquake.
What makes the November recordings weird are their long, monochromatic signal. “At most seismometers around the world the signal is long and almost entirely at one period—17 seconds—without the typical, higher frequency P- and S-waves emitted by most earthquakes,” Lomax noted.
So far nothing has been definitively connected to the event.
Mayotte is among an archipelago of volcanic islands called Comoro. The island itself has two volcanoes that have remained dormant for more than 4,000 years. However, National Geographic noted that hundreds of tremors have swarmed Mayotte since May 2017, and the French Geological Survey is currently monitoring the region for new volcanic activity.