Back on Nov. 11, seismographs from around the world registered a mysterious tremor that encompassed the planet. As of right now, scientists still don't know what caused it, reports National Geographic.

It's not just the source of the rumble that remains mysterious, it's also the form of the wave, which was characterized as a monotone, low-frequency “ring,” and which was registered by instruments but felt by no one.

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

The biggest clue we have so far is an ongoing seismic swarm that has been occurring just offshore from the archipelago of Mayotte, which sits about halfway between Africa and Madagascar. The Nov. 11 wave looks to have originated from around there. The only problem is, the Mayotte seismic swarm is also something of a mystery.

The swarm is composed of hundreds of tremors that began around May 10th and have yet to disappear. While they are numerous, they aren't particularly large events; the largest quake of the bunch came in at a 5.8 magnitude. It's not exactly 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; it probably came from some combination of tectonic and volcanic processes, and possibly other deep Earth processes. It also might be 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 is not ordinarily known for its seismic activity, scientists don't have a lot of instruments currently at play from which to draw data. 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.

Plans are already underway to do ocean bottom surveys to get more detailed information about the region and investigate the possibility of a submarine eruption, but until those investigations are complete, it's anyone's guess what's going on. Hopefully we'll have some more insight before any more seismic waves wash over the planet.

What caused a mysterious seismic wave to wash over our whole planet?
On Nov. 11, seismographs from around the world registered a mysterious tremor that encompassed the planet.