The event was originally detected by seismic sensors on an island between Madagascar and Africa. Then, from as far afield as Chile, New Zealand, and Canada, alarm bells began to sound.

The “event”, which went on for about 20 minutes on November 11, 2018, was also picked up by Hawaii, which is nearly precisely on the opposite side of the earth. Nobody knows what it was. Meteorite? Volcano under the sea? Nuclear detonation?

“I don’t think I’ve seen anything like it,” National Geographic reports Columbia University seismologist Göran Ekström as saying. “It doesn’t mean that, in the end, the cause of them is that exotic.”

The little island of Mayotte, located about midway between Africa and Madagascar, lies at the heart of the enigma. Since May, it has been hit by a swarm of earthquakes. Most have been mild, but the greatest, on May 8, was the largest in the island’s recorded history, measuring 5.8 on the Richter scale.

However, the seismic swarm was in decline when the odd ringing was discovered early November. Ekström, who specializes in unusual earthquakes, observes that much about the November 11, 2018 occurrence was odd. It seemed as though the planet rang like a bell as it expanded, keeping a low-frequency monotone.

Earthquakes, by definition, register as brief, sharp “cracks.” When stresses in the Earth’s crust abruptly release, bursts of plainly discernible seismic waves spread outward from the location of the slippage.

The initial signal is referred to as a primary wave, and it consists of high-frequency compression waves that radiate in clusters. Then there’s the Secondary wave, which has a higher frequency and tends to “wiggle” more.

Then come the surface waves, which are sluggish, deep rumbles that may circle the Earth multiple times. The November 11 incident is remarkable for the absence of any major or secondary waves. The only thing that registered was a deep, resonant surface wave. And it didn’t rattle like an earthquake’s surface wave. Instead, it kept a much clearer – almost melodic – frequency.

According to National Geographic, the French Geological Survey believes a new volcano is forming off the coast of Mayotte. The island was formed by volcanic activity, although it has been inactive for about 4,000 years.

The strange ringing is thought to have been caused by magma movement 30 miles off the coast and deep underground, according to the French. GPS sensors have detected that Mayotte had migrated 2 inches to the southeast in less than five months. However, it is a poorly charted area. It’s impossible to know exactly what’s under the sea.

Ekström suspects that the extremely clean signal was created by magma swirling about within a chamber or being driven through a crack in underlying rocks.

But he’s not sure.