Earlier this month, seismic stations from Madagascar to Canada picked up mysterious waves that rumbled for more than 20 minutes, unbeknownst to most people.
Researchers and earthquake enthusiasts who have spotted the signals have narrowed the origin to a region just off the coast of the island of Mayotte, in the Indian Ocean.
But, no one's pretty sure what caused the strange event.
The slow waves detected from Mayotte on November 11 are those typically seen after the great earthquakes, and they are known to travel great distances – but, no such earthquake took place.
Researchers and earthquake enthusiasts who spotted the signals have narrowed down the origin to a region just off the coast of Mayotte Island (shown), in the Indian Ocean. But, no one's pretty sure what caused the strange event
"I do not think I've ever seen anything like it," Göran Ekström, a seismologist at Columbia University who specializes in unusual earthquakes, told National Geographic.
But, he adds, 'It does not mean that, in the end, the cause of them is that exotic.'
A Twitter thread that garnered attention from the seismology community first revealed the strange phenomenon on the morning of Nov. 11.
A Twitter thread that garnered attention from the seismology community first revealed the strange phenomenon on the morning of Nov. 11. Scientists and earthquake enthusiasts alike worked to narrow it down
'This is a odd and unusual seismic signal,' Twitter user @matarikipax wrote alongside a seismograph reading from Kilima Mbogo, Kenya.
Over the course of that day, others may have been in the conversation to point out where else the low-frequency waves were detected: Chile, New Zealand, Canada, and Hawaii.
The signals generated by the waves from Mayotte came up clean, with a zigzagging pattern primarily of one type of wave, which took 17 seconds to repeat, according to National Geographic.
Waves like this are known as monochrome.
The strange waves were traced to an origin roughly 15 miles off of the French island, Mayotte
Scientists are working to understand what spurred the mysterious waves on that day. So far, many suspects are related to an ongoing seismic swarm in the region that started in May.
But even then, there was no corresponding earthquake on Nov. 11.
Researchers with the French Geological Survey (BRGM) say it could be a signal that the magma beneath the volcanic island is shifting offshore.
Others say there has been a 'slow' earthquake that simply went unnoticed, or an underwater eruption.
'Confirmation of location places it near the Comoros', researchers who made the find says.
Experts say the complex geology of the region is still the issue, potentially filtering some of the waves to make the clean signal, NatGeo reports.
Scientists plan to survey the ocean to find out any additional information that could help explain the mysterious phenomenon.
But at this stage, experts agree there's just too much we do not know to say what was really to blame.
HOW ARE EARTHQUAKES MEASURED?
The magnitude of a earthquake differs from its intensity.
The magnitude of a earthquake refers to the measurement of the energy released when the earthquake originated.
Magnitude is calculated based on measurements on seismographs.
The intensity of a earthquake refers to how strong the shaking that is produced by the sensation is.
A 5.3 magnitude earthquake hit the Channel Islands off the coast of southern California on Thursday at 10.30am
According to the United States Geological Survey, 'intensity is determined from the effects on humans, human structures and the natural environment'.
Earthquakes originated below the surface of the earth in a region called the hypocenter.
During a earthquake, one part of a seismograph remains stationary and one part moves with the earth's surface.
The earthquake is then measured by the difference in the positions of the still and moving parts of the seismograph.