The volcano came into the world wailing, but for a while nobody heard it.

It was born in the summer of 2018 just off the coast of the tiny island of Mayotte, a French territory halfway between Madagascar and Mozambique. An earthquake swarm in May of that year precipitated its arrival like a drum roll. Magma rumbled from within a reservoir at the top of the Earth’s mantle. The magma migrated up through the crust, sending tremors across the nearby island as it moved — until finally, sometime in late June or early July, with no precise birth date yet recorded, it popped its head out of the ocean floor.

For months, the underwater volcano announced its own birth with mysterious cries: a low seismic humming too faint to feel. It wasn’t until Nov. 11, 2018, that anyone noticed. Something strange happened that day. The seismic waves traveled all over the world, to Kenya and Chile, Canada and Hawaii, nearly 11,000 miles away. And the humming got louder, longer, lasting up to a half-hour.

“This is a most odd and unusual seismic signal,” a New Zealand earthquake enthusiast wrote on Twitter while linking to a U.S. Geological Survey seismograph.

The post caught the attention of seismologists worldwide, as they tried to pinpoint the source of the bizarre, droning frequency.

It was coming from the coast of Mayotte, they soon learned — and now a team of German geoscientists has pieced together exactly why.

In a paper published this week in the journal Nature Geoscience, the researchers have deciphered the tumultuous events in the depths of the Indian Ocean that brought the Mayotte volcano into being, tracing the drainage of the “exceptionally deep” magma reservoir up to the ocean floor while discovering the cause of the mysterious humming.

Simone Cesca, a seismologist at the GFZ German Research Center for Geosciences and the paper’s lead author, told The Washington Post Thursday that this marks “the first time we’ve really observed the birth of a volcano on the sea floor.” Last year, a team of French seismologists were the first to confirm the existence of the new submarine volcano.

“The whole episode is really, really rare,” Cesca said. “Seeing the deep magma chamber, seeing the magma’s propagation to the surface, seeing the volcano being born — I think this is unique, absolutely.”

Cesca said what his team tried to do was compile the observations made by numerous geologists over the past 18 months and connect all the dots, starting with the earthquakes.

Mayotte, a volcanic island in the Comoros archipelago home to more than 250,000 people, has not experienced a volcano eruption in approximately 4,000 years, according to the Nature Geoscience paper. Earthquakes with a magnitude over 4.0 have only been recorded several times in history in the region, according to the paper — which is why the series of large earthquakes in May 2018, peaking at a magnitude of 5.9, caught seismologists’ attention. It was the largest earthquake ever recorded on Mayotte.

Eleonora Rivalta, a physicist who studies earthquakes and volcanoes at the German Research Center for Geosciences, said her team began its research that May. A volcanologist on the research team had a sister in Mayotte and worried for her safety, she said.

The earthquakes, they soon learned, were a symptom of something much bigger.

Deep in the crust of the Earth, the magma was stirring. “A pocket of magma decided it wanted to erupt,” Rivalta said, and so it started heading for the surface — the sea floor. According to the scientists’ estimate, it is one of the deepest magma chambers ever discovered, approximately 16 to 19 miles deep. “Once you create a channel to the surface, then the magma starts to pour out and create the volcano,” Rivalta said. “This is the cause of everything.”

The magma chamber started to drain as the lava moved toward the ocean floor. As the chamber became increasingly hollow, its roof started to cave, Rivalta said.

Then came phase two: the mysterious humming — the quiet earthquakes you couldn’t feel.

“Every time the rock sags into the chamber, it creates a resonance,” Cesca said, “and this produces this strange signal that you see far away.”

The seismologists recorded 407 unusual signals coming from the site of the magma chamber near Mayotte, and nearly 7,000 earthquakes of varying intensity, most of which could not be felt on land. The seismic humming started in June, according to data collected in the paper, before finally being picked up Nov. 11, 2018.

Stephen P. Hicks, who studies earthquake seismology at the Imperial College of London and who is unrelated to the team of Mayotte researchers, said he does not believe scientists could have made the discovery of the underwater volcano so quickly if the Nov. 11 signal not been detected.

In May, the team of French researchers discovered the enormous newborn volcano, three miles in diameter, rising 800 meters (about half a mile) from the sea floor. “We have never seen anything like this,” the Nathalie Feuillet of the Institute of Geophysics in Paris, one of the French research organizations, told Science magazine.

The discovery led many scientists to suspect the seismic hums must have originated from the volcano. Cesca says his team is the first to confirm the connection, while identifying the more exact cause: the magma chamber’s sagging roof.

Hicks said a “unique aspect of this study is it shows how quickly the magma can rise and create either a new volcano or an eruption.

“This paper gives us a framework to interpret these seismic events,” he said. “The amount of magma that moved might have been the greatest amount ever observed.”

Multiple teams of French researchers have papers forthcoming on their findings, and one was already preprinted last year but not yet peer-reviewed. The preliminary report echoes the findings of the German team.

For now, Cesca said he suspects the young volcano is just about done growing, as most of the magma that built the volcano has probably drained to the surface. But the chamber is so deep inside the Earth’s core, he said, “we never know.”