(ZME Science) – Something was brewing underneath the Comoros archipelago, and the Earth was rumbling. All around the world, researchers have installed seismometers that captures the Earth’s minutest vibrations. When an earthquake takes place, it might rumble the area closest to it — but echoes of this rumbling are spread all around the world and can be detected by precise equipment.

This is actually how we know what the interior of the planet looks like: vibrations spreading from one point on the Earth to the other are affected by the environment they travel in, and they carry “fingerprints” of these environments. So when researchers picked up an unusual “humming” coming from the inside of the Earth, they took it very seriously.

It all started with an unusual amount of earthquakes from the island of Mayotte in the Indian Ocean — one of the areas in the Comoros archipelago between Africa and Madagascar. Over 7,000 earthquakes were detected, the most severe of which had a magnitude of 5.9. To make matters even more mysterious, some earthquakes exhibited an unusual type of oscillations: low-frequency and almost-harmonic vibrations, almost like those from a large bell. FULL REPORT