The seismic sensors have for the first time detected the event near an island located between Madagascar and Africa. Then, alarm signals sounded all the way to Chile, New Zealand and Canada.
Hawaii, almost exactly on the other side of the planet, also participated in "the event".
Nobody knows what it was.
Meteorite? Underwater volcano? Nuclear test?
"I do not think I have seen anything like it," said Göran Ekström, a seismologist at Columbia University. "That does not mean that, ultimately, their cause is so exotic."
At the center of the mystery is the tiny island of Mayotte, located about halfway between Africa and Madagascar. It has been subjected to a swarm of earthquakes since May. Most were minor, but the most important – May 8 – was the largest in the history of the islands, with a magnitude of 5.8.
But the swarm of earthquakes was declining before the mysterious ring was detected earlier this month.
Ekström, a specialist in unusual earthquakes, points out that the November 11 event was strange. It was as if the planet sounded like a bell, maintaining a low monotonous frequency during its spread.
Earthquakes, by their very nature, tend to be very short "cracks".
When tensions in the earth's crust are abruptly dissipated, clearly identifiable seismic waves radiate to where the landslide occurs.
The first signal is called the primary wave: a high frequency compression wave that emits packets.
Then comes a secondary wave: these high-frequency waves tend to "move" more.
It is only then that the surface waves come: these slow and deep rubs tend to linger and can go around the Earth several times.
The event of November 11 is remarkable in that no primary or secondary wave was detected.
All that was recorded was the deep, resonant surface wave. And that did not happen as the surface wave of an earthquake does. Instead, he maintained a much cleaner frequency – almost musical.
National Geographic reports that the French Geological Survey suspects a new volcano to develop off Mayotte. Although the island was created by volcanic activity, it has been dormant for more than 4,000 years.
The French think that this strange ringing may have been caused by a movement of magma some 30 miles from the coast and under deep water. This is corroborated by GPS sensors detecting that Mayotte has moved about 2 inches to the southeast in less than five months.
But it is a poorly mapped region. One can only guess exactly what lies beneath the ocean.
Ekström thinks that the unusually pure signal could have been caused by the slippage of magma inside a room or by the forced passage of a space in the underground rocks.
But he is not sure.