Strange waves rippled around Earth. Now we may know why.

On May 10, 2018, the geologic beasts of the tiny island of Mayotte began to stir. Thousands of earthquakes rattled the French island, which is sandwiched between Africa and Madagascar. Most were minor shakes, but they included a magnitude 5.8 event that struck on May 15, the largest yet recorded in the region's history.

In the midst of this seismic swarm, a strange low-frequency rumble rippled around the world, ringing sensors nearly 11,000 miles away—and baffling scientists.

Now, researchers may have at last found the source of the unexpected activity: the birth of a submarine volcano some 31 miles off Mayotte's eastern shore. Sitting about two miles underwater, the baby volcano stretches nearly half a mile high and extends up to three miles across.

The observations came after French scientists launched a multi-pronged mission to get a better grip on the origin of the ongoing seismic swarm. Coordinated by France's National Center for Scientific Research (CNRS), the work includes surveys from the ship Marion Dufresne co-led by Nathalie Feuillet from the Institut de Physique du Globe de Paris (IPGP) and Stephan Jorry of the French research institute IFREMER. The latest venture also retrieved six underwater seismometers that have been listening for earthquakes since February.

The data are still preliminary, and the scientists are currently working to analyze their findings and publish the research in a peer-reviewed journal. In the meantime, the team has issued a joint press release announcing the new volcano and its probable link to the odd throng of earthquakes.

“In light of this discovery, the government is fully mobilized to pursue and deepen our understanding of this exceptional phenomenon and take necessary measures to categorize and prevent any risks it represents,” the agencies say in the release.

Stephen Hicks, a seismologist at Imperial College who previously analyzed Mayotte's strange seismic happenings, adds that the announcement offers some much needed clarity for the island's inhabitants, who have been thoroughly shaken after months of unexplained tremors.

The geologic mystery

Mayotte is part of the Comoro archipelago, a string of volcanic islands northwest of Madagascar. While volcanism isn't unheard of in the region, Mayotte has long remained silent, with its last eruption bursting free more than 4,000 years ago. But starting in May of last year, the geologic activity on Mayotte kicked into high gear. Since the swarm began, more than 1,800 earthquakes greater than magnitude 3.5 have rattled the tiny island. The land itself also seems to be on the move, drifting 0.6 inches east and sinking roughly 0.4 inches down each month since mid-July.

In November, the curious low-frequency rumbles began their global spread, sticking around for more than 20 minutes. Too low of a frequency for humans to feel, only one person noticed the curious waves: An earthquake enthusiast who goes by the handle @matarikipax spotted the unusual zigzags on the U.S. Geological Survey's real-time seismogram displays and posted them on Twitter, drawing an international cohort of scientists to the mystery.

“It was clearly a both concerning and fascinating event that was happening,” says marine seismologist Wayne Crawford of IPGP, who was part of the recent expedition. “It was something we’d never seen before.”

Even back then, the experts' conclusion was that the quakes and strange seismic signal were likely related to the movement of molten rock. Perhaps the earthquake swarm was the result of magma squishing through the subsurface, and the low-frequency rumble was caused by waves resonating in a collapsing magma chamber.

The link to volcanic activity gained further support from a preprint study posted to the EarthArxiv server in February 2019. That research pinned the swarm on a massive magma chamber starting to drain, in what could be the largest off-shore submarine volcanic event yet documented.

But with limited monitoring of these earthquakes near their epicenters out at sea, and no direct evidence of an eruption, nothing more definitive could be said at the time.

Pregnant Earth

Then, on May 16, the French collaboration issued their press release, and Robin Lacassin of the Institut de Physique du Globe de Paris, one of the organizations involved in the research, posted a pair of images on Twitter. One picture shows the newborn volcano as seen via acoustic imaging, which acts somewhat like a dolphin using sonar to sense its surroundings.

“It's almost a pregnancy ultrasound ... only with larger error bars,” said geophysicist Lucile Bruhat, who was not part of the research team, about the picture on Twitter.

In the image, a twisting plume rises 1.2 miles through the water column from the top of a conical edifice. Exactly what this plume is made of remains unknown, but the sound waves might be bouncing off glassy shards similar to the ash that billows out of erupting volcanoes on land, notes Helen Robinson, a Ph.D. candidate in applied volcanology at the University of Glasgow, via email. But even temperature and density differences of the water would show up in the images, Crawford adds, so the plume could just be a hot mineral-rich stream, like the roiling waters from hydrothermal vents.

While the volcano is definitely young, exactly how young remains up for debate. It was absent in seafloor maps of the region drawn up in 2015—and the team thinks that it didn't exist prior to the onset of earthquake activity last May, Crawford says. Its birth could be as recent as the summer of 2018, when GPS sensors tracked the island sinking and shifting east as, presumably, magma drained from a chamber below.

“What we know for a fact is that that thing didn’t exist in 2015, and now it’s here,” he says.

The other image reveals a series of bumpy structures on the seafloor that seems to form a loose path to the new volcanic center from where the most recent earthquakes are rattling, between three and nine miles offshore Mayotte's Petite-Terre island. Even in the area where the new volcano is forming, abundant ridges and bumps reveal past eruptions that could have emerged many years ago, Crawford says.

“Maybe that volcanic center has migrated away from the island itself,” Hicks speculates, but he notes that more data are needed to date these potential spots of volcanic activity and confirm this mechanism.

Crawford agrees that the features seem to be volcanic. And by using the new earthquake data to recalibrate the old, a curious pattern emerges, he says. The quake swarm seems to split into three activity centers along the ridge of bumpy structures: one under the new volcano, the most recent roughly six miles offshore, and a third one halfway in between. But how these regions are connected is unknown, Crawford says.

There are a few similarities between this new structure and Hawaii's Lō'ihi seamount, an underwater volcano growing south of Kīlauea, adds Ken Rubin, a volcanologist at the University of Hawaii at Mānoa who specializes in underwater eruptions.

Hawaii is formed from what's known as hotspot volcanism; each island represents a boil that formed above a deep plume of molten rock, popping up in a chain as the overlying tectonic plate inches along. Lō'ihi is the youngest of these volcanoes. In 1996, it burst into a fit of unrest, generating thousands of quakes similar to what's been seen around Mayotte, Rubin says. For Lō'ihi, the pulse of activity was the result of magma draining from a reservoir that caused the empty chamber to collapse.

In the Comoros, however, the situation is a bit more complicated. Some geologists believe that the volcanic chain there comes from similar hotspot activity. But the archipelago also sits within an ancient rift—the gaping wound where Madagascar tore away from eastern Africa—and volcanic activity is possible along fissures that formed from this break. Curiously, the latest activity boiled off the shores of Mayotte, which is the oldest in the island chain, Rubin notes.

Clues to the source of the latest volcanic activity may be locked in the minerals of the solidified lava on the seafloor, Hicks adds. The team nabbed samples of these rocks from the flanks of the baby volcano, so studies of this material and of the surrounding region can help researchers start piecing together the picture.

Pure and exciting

So how exactly is the new volcanic activity linked to the seismicity, including the strange low-frequency signal?

“That’s the million dollar question,” Hicks says.

Research presented at a recent European Geosciences Union meeting revealed that the long, low-frequency signal noted in November wasn't the only event of its kind at Mayotte, he says. Instead, it seems to be a common feature of this ongoing swarm of quakes. But scientists can't yet say what precise situation is causing the low-frequency signals and the seismic swarm—or even if the new volcano's eruption is ongoing.

“There’s still so much research to do,” says Mark Tingay, a specialist in geomechanics at the University of Adelaide via Twitter direct message. “But it’s an opportunity for scientists to study what is possibly the birth or reawakening of a submarine volcano.”

Still, having a promising lead in the case has been a boon for the people around the world who have been following the quake swarm through social media. Researchers have been posting updates as the work progressed, offering a peek into “science in its most pure and exciting form,” as Wendy Bohon of the Incorporated Research Institutions for Seismology says in an email.

And of course figuring out the mystery has been of great importance for the island's residents. The uncertainties surrounding the source of the quakes and a lack of adequate communication early on during the slew of ground-rattling events caused both frustration and confusion among the locals. That in turn sparked a range of wild theories about what might be behind the moving earth, says Laure Fallou, a sociologist with the Euro-Mediterranean Seismic Centre who has studied the role of culture in effective science communication in the region.

The recent announcement instead brings a new wave of emotion: “They went from the fear to the fascination,” Fallou says. “Something incredible was happening there.”