Scientists have been left somewhat bemused by seismic waves traveling around the world, because no one is completely sure what caused them.

As reported by National Geographic, these waves were detected on November 11 by sensors around the world. Believed to have originated near the French island of Mayotte between Madagascar and Africa, they were detected in Zambia and Kenya, and then made their way to Canada, Chile, New Zealand, and even Hawaii.

Despite the waves lasting for 20 minutes, no person felt them. Instead, their presence was spotted on a real-time seismogram from the US Geological Survey. And scientists were immediately confused as to what was causing them.

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

These waves were thought to be particularly slow and low frequency, pulsating every 17 seconds during the course of the event. Exactly what caused them, however, is a bit of a mystery – although there are some ideas.

The French Geological Survey (BRGM), who are investigating the event, say it could have been the result of new volcanic activity off the coast of Mayotte. It’s also possible that an earthquake triggered the rumblings, or perhaps some sort of eruption in the water.

“According to one analysis, this movement could be due to the emptying of a magma reservoir nearby, although additional research would be needed to verify this,” noted Science Alert.

Interestingly, half a year before this event scientists had detected a swarm of earthquakes off the east coast of Mayotte, which had continued up to this latest event and have been causing the island to move very slightly. But this didn’t appear to have the same characteristics as a regular earthquake.

“These observations therefore back up the hypothesis of a combination of tectonic and volcanic effects accounting for a geological phenomenon involving a seismic sequence and a volcanic phenomenon,” the BRGM said in a post.

Upcoming surveys are planned to try to get to the bottom of the event, while scientists are continuing to pore through the seismic data that’s available. Of course, there’s always the possibility this was a mistake in the detections, too. We’ll just have to wait to find out.

[H/T: National Geographic]