Back on Nov. 11, seismographs from around the world registered a mysterious tremor that encompassed the planet. Now scientists think it may have been caused by the largest offshore volcanic event in recorded history.

A report by scientists at the French Geological Survey and France’s Ecole Normale Supérieure suggests a massive movement of magma that caused the seafloor to deflate, reports Gizmodo.

Their investigation is explained in a yet-to-be-peer-reviewed report submitted to EarthRXiv.

The November event off the coast of southeast Africa near the island of Mayotte is what grabbed the attention of many scientists. It wasn't just the source of the rumble that was mysterious, but also the form of the wave, which was characterized as a monotone, low-frequency "ring" that registered on instruments but was felt by no one, according to National Geographic.

"I don't think I've seen anything like it," seismologist Göran Ekström from Columbia University said at the time.

The swarm was composed of hundreds of tremors that began around May 10, 2018, and is ongoing. According to the new report, there were 29 earthquakes with a magnitude greater than 5 that occurred during that window. The largest quake of the bunch came in at a 5.8 magnitude.

It's not something you might expect to cause a seismic wave that's measured around the world. It's worth noting, however, that the 5.8 quake was the largest ever recorded in the region.

Something new is going on here

One thing we know is that tectonic activity alone probably can't account for the wave. This might mean that a new center of volcanic activity is developing off the Mayotte coast, a shift in a nearby magma reservoir, which would make these events a fascinating opportunity for research.

Whatever the cause, there's no doubt that something new is going on here. The Mayotte archipelago is already being radically transformed by the events, at least in geological terms. For instance, the island appears to be on the move. It has moved about 2.4 inches to the east and 1.2 inches south since the seismic swarm began.

Because the region isn't ordinarily known for its seismic activity, scientists don't have a lot of answers. That's another big reason for the mystery.

“It is very difficult, really, to say what the cause is and whether anyone's theories are correct," said Helen Robinson, a Ph.D. candidate in applied volcanology at the University of Glasgow.

Research in the area is ongoing. Hopefully we'll have some more insight before any more seismic waves wash over the planet.

Editor's note: This story has been updated since it was originally published in November 2018.

A mysterious seismic wave washed over the planet, and now we think we know what it was
In November 2018, seismographs from around the world registered a mysterious tremor that encompassed the planet. Now we think it was a massive volcanic event.