Mysterious Planetwide Rumble May Have Come From the Largest Underwater Eruption Ever Recorded

On November 11, 2018, a deep rumble ricocheted around the world, one that humans couldn’t feel but that registered quite clearly on seismometers.

It happened near the French Island of Mayotte, near Madagascar.

Mayotte, as it is now.

Originating 30 miles east of the island of Mayotte, near Madagascar, the mid-November signal immediately caught the attention of a disparate group of geoscientists. They subsequently took to Twitter to express their fascination over this mysterious event—one even joked about a “giant prehistoric sea monster.”

The rumble formed part of a prolonged seismic sequence that had started in the area back in May 2018, but the very low-frequency, potent growl in November stood out because it wasn’t immediately obvious what caused it. These scientists eventually agreed that it could only have originated from a volcanic event, one involving the movement of a vast volume of magma beneath the seafloor, causing the ground there to significantly deflate.

There is a major caveat to all this, however. Compared to land-based monitoring, there’s a huge lack of offshore monitoring happening around the world today, and there are likely plenty of offshore events that have taken place since modern records began that scientists haven’t picked up on.

Make no mistake, though: The recent event offshore from Mayotte, which is still ongoing, is colossal. 

According to the data from the onshore GPS stations, as well as the seismic signals—including the weird November 11 event—the rumbling is definitely being generated by volcanic activity of some sort. The way the ground on Mayotte is moving implies that the seafloor off its eastern shoreline is sinking at a rate of around 0.4 inches per month. At the same time, Mayotte itself is shifting eastward at a rate of 0.63 inches per month. Both indicate something huge underground is on the move, causing some serious deflation.

The nature of these tremors suggest that the magmatic source is centered at a depth of 16 miles beneath the seafloor. In the first six months of the sequence alone, at least 0.24 cubic miles of magma has shifted around. That’s roughly equivalent to 385 Great Pyramids of Giza.

What could happen next?  No one knows, but we will in fact see!