On May 10, 2018, the geologic beasts of the tiny island of Mayotte began to stir. Thousands of earthquakes rattled the French island, which is sandwiched between Africa and Madagascar. Most were minor shakes, but they included a magnitude 5.8 event that struck on May 15, the largest yet recorded in the region's history.

In the midst of this seismic swarm, a strange low-frequency rumble rippled around the world, ringing sensors nearly 11,000 miles away—and baffling scientists.

Now, researchers may have at last found the source of the unexpected activity: the birth of a submarine volcano some 31 miles off Mayotte's eastern shore. Sitting about two miles underwater, the baby volcano stretches nearly half a mile high and extends up to