Wait, did anyone else hear that? Back in 2018, hundreds of earthquake monitoring stations all over the world started picking up on an unusual “hum” that rumbled throughout the planet. After months of confusion and curiosity, a team of researchers has pinpointed the source: the birth of an underwater volcano.

The story begins in May 2018 when seismologists detected an unusual chain of earthquakes off the coast of Africa, culminating in an earthquake of magnitude 5.9. Things got stranger in November 2018 when a number of unusual 20-minute-long "hums" were registered on sensors across Africa, South America, New Zealand, North America, and even Hawaii. 

"Something biggggg, yet strangely slow, sent seismic rumblings around the surface of much of the planet yesterday," Stephen Hicks, a seismologist at the University of Southampton, tweeted on November 12, 2018.

“I’m rooting for the event to be a giant prehistoric sea monster,” joked Dr Mark Tingay, an associate professor of seismology at the University of Adelaide. “Submarine volcanism sounds most likely.”

Writing in the journal Nature Geoscience this week, scientists led by the GFZ German Research Centre for Geosciences in Germany took a deep look at the hundreds of signals to pinpoint the source of this “deep, rare magmatic process.”

Just as other research had speculated, their workings led them to an underwater volcano that formed in 2018 off the coast of Mayotte, an archipelago in the Indian Ocean between Madagascar and the coast of mainland Africa. Located some 3,500 meters (11,500 feet) beneath the water’s surface, the volcano is estimated to be 800 meters (2,624 feet) tall with a base of 4 to 5 kilometers (2.4 to 3.1 miles) in diameter. 

The underwater eruption resulted in more than 3.4 cubic kilometers (0.8 cubic miles) of magma to gush out of the seafloor, marking what they described as the largest underwater eruption ever recorded to date.

The hums were seismic waves echoing out of the movement of magma as it traveled diagonally up through Earth’s crust from a reservoir 35 kilometers (22 miles) under the seabed. Through this new work, the team was able to detail each of the different stages of this process (diagram above), including the drainage of the reservoir, which they believe is one of the deepest and largest active magma reservoirs ever discovered in the upper mantle.

While this all sounds like a momentous event, it was scarcely noticed around the island itself. 

"Since the seabed lies 3 kilometres below the water surface, almost nobody noticed the enormous eruption. However, there are still possible hazards for the island of Mayotte today, as the Earth's crust above the deep reservoir could continue to collapse, triggering stronger earthquakes," Torsten Dahm, head of the section Physics of Earthquakes and Volcanoes at the GFZ, said in a statement.