A series of seismic waves that rolled around the planet earlier this month is baffling scientists.
On November 11, seismic activity was picked up by monitoring stations starting near Africa and traveling as far away as Canada and New Zealand.
But there were no associated earthquakes, causing confusion over exactly what had caused the rumble.
It lasted for about 20 minutes but no one felt it above ground.
"I don't think I've seen anything like it," seismologist at Columbia University Goran Ekstrom told National Geographic, adding that he specialises in unusual seismic activity.
While GeoNet seismologist John Ristau told New Zealand's stuff.co.nz "it is a very strange signal and it can be seen pretty much everywhere around the world".
"The signal is clearly not like a regular earthquake, it's more like a burst of energy," he said.
It's sent both scientists and social media into a frenzy of speculation about the source of the waves, ranging from submarine volcano to nuclear test to meteorite strike to sea monster.
The activity is believed to have started near Mayotte, a French island between continental Africa and Madagascar.
Hundreds of small earthquakes have rattled the area over the last year, yet the frequency of the shakes has declined over recent months. There was no major quake there on November 11.
And while earthquakes register short-sharp "cracks" the November 11 event appeared to be more of a "surprisingly monotone, low-frequency" buzz according to National Geographic.
The outlet called the features "remarkably weird".
An earthquake enthusiast in New Zealand was one of the first people to notice the unusual activity.
Under the twitter handle @matarikipax, the enthusiast shared readings from the US Geological Survey, which are published free online.
"This is a most odd and unusual seismic signal. Recorded at Kilima Mbogo, Kenya."
Stephen Hicks, a seismologist at the University of Southampton, later tweeted "something biggggg, yet strangely slow, sent seismic rumblings around the surface of much of the planet yesterday".
However, Mr Ekstrom made the point that "it doesn't mean that, in the end, the cause of them is that exotic".
So far, at least one expert seems to think he has solved the mystery.
Independent seismology consultant Anthony Lomax told the Daily Mail the activity was "almost certainly" caused by undersea volcanic activity northeast of Mayotte.
"There has been ongoing low-level seismic activity there since May," he said.
"Inflation/deflation and collapse of volcano calderas, and movement of magma under a volcano can produce a wide variety of seismic signals, including long period and repetitive waves like those observed November 11."
