Scientists studying our planet have a pretty good idea of what goes on up here at the surface. Understanding what’s going on deep underground or in the depths of the Earth’s oceans is another matter entirely. Without being able to constantly observe the processes happening miles beneath our feet, researchers often have to listen instead.

Two years ago, many of the instruments used to listen in on the inner workings of the Earth began acting, well, odd. All over the world, some of the most sensitive seismic tools revealed a bizarre “hum” coming from deep within the planet. At the time, scientists could only guess what was happening, but a new paper published in Nature Geoscience offers a more concrete explanation.

The strange humming noise came in bursts, radiating out for hundreds of miles and lasting as long as a half-hour. An abundance of seismic activity was detected in the Indian Ocean, near the island of Mayotte, and researchers focused their attention on the ocean floor around the island. The island itself actually “sunk” by over six inches, and researchers now say that the birth of a massive new underwater volcano is responsible.

The birth of the volcano and the movement of tectonic plates in the area resulted in approximately 7,000 earthquakes, though the vast majority of them were too small to be noticeable.

The volcano itself is huge, stretching around 800 meters from the ocean floor. Comparisons of the seafloor from months prior reveal that it was indeed created while the strange humming was documented.

By combining the seismic readings with observations of the seafloor, the team was able to estimate the size of the magma reservoir that drained during this period. “We demonstrate that such deep offshore magmatic activity can be captured without any on-site monitoring,” the researchers write.