Seismic sensors are the first event originating near an island between Madigascar and Africa. Then, alarm bells start ringing as far away as Chile, New Zealand and Canada.
Hawaii, almost exactly on the other side of the planet, also picked up the 'event'.
Nobody knows what it was.
Meteorite? Submarine volcano? Nuclear test?
"I don't think I've seen anything like it,"National Geographic reports Colombia University seismologist Göran Ekström as saying. "It doesn't mean that, in the end, the cause of them is that exotic."
At the center of the mystery is the tiny island of Mayotte, positioned about half way between Africa and Madigascar. It's been subjected to a swarm of earthquakes since May. Most have been minor, but the biggest – on May 8 – recorded history, topping magnitude 5.8.
But, the earthquake has been in decline before the mysterious ringing was detected earlier this month.
Extröm, who specializes in unusual earthquakes, points out much about the November 11 event was weird. It was as though the planet rang like a bell, maintaining a low-frequency monotone as it spread.
Earthquakes, by their very-nature register, are short-sharp as cracks ’. As in Earth's crust suddenly release, pulses of radiate outward from where the slippage occurs.
The first signal is called a Primary wave: high-frequency compression waves that radiate in bunches.
Secondary wave: these high-frequency waves tend to be 'wiggle' somewhat more.
Only then comes the surface waves: these slow, deep rumbles linger, and can circle the Earth several times.
The November 11 waves were detected.
All registered are deep, resonate surface waves. And it didn’t ‘rumble’ as an earthquake’s surface wave tends to. Instead, it is maintained a lot cleaner – almost musical – frequency.
National Geographic reports the French Geological Survey suspects a new volcano may be developing off the coast of Mayotte. While it was created by volcanic activity, it's been dormant for more than 4000 years.
The French is believed to be weird ringing, it has been generated by a movement of some 50km off the coast and under deep water. This is supported by GPS sensors detecting Mayotte has moved some 5cm to the southeast in less than five months.
But it's a poorly mapped region. Exactly what’s beneath the ocean’s can only be guessed at.
Ekström believes that the unusually pure signal could have been caused by the magma sloshing about inside a chamber, or being forced through a gap in the subsurface rocks.
But he’s not certain.