The deepest earthquake ever detected shouldn't have been possible, scientists say, at least according to what they currently know.
The quake happened in 2015, striking Japan's Bonin Islands. It was a relatively minor aftershock to an earlier 7.9 quake, at least on the surface - undetectable to anyone without sophisticated seismic monitoring equipment.
Scientists knew the quake was deeper than usual, but just how deep it was - revealed in a recent scientific paper - has come as a complete shock.
If the calculations are correct, it was 751km down, deep into the Earth's lower mantle and well below the tectonic plates, which sit on the crust and cause the usual kinds of earthquakes we're familiar with.
In comparison, the 2016 Kaikōura earthquake was 9km deep and the 2011 Christchurch quake just 4km.
National Geographic reports only about one in 20 quakes are measured more than 300km beneath the planet's surface, but how they occur has long been a mystery. That far deep, rock is hot, soft and malleable - meaning although it's under extreme pressure it tends to bend, rather than break, and quakes should be "unlikely, if not impossible".
One scientist told Live Science they've worked out how quakes down to about 400km happen - pressure forces fluids out of pores in rocks, making them more brittle. But below that, no one's quite sure.
"At that depth, we think all of the water should be driven off, and we're definitely far, far away from where we would see classic brittle behavior," said Pamela Burnley, a professor of geomaterials at the University of Nevada.
A few different explanations have been suggested - including a piece of sinking crust that may have pierced the mantle, its cold temperature upsetting pressure balances in the hotter mantle; another is that the extreme pressures that far down might cause minerals to break down in unexpected ways, creating weak points that could lead to sudden slips.
"We can't get down there," University of Southern California geophysicist Heidi Houston told National Geographic. "We only see what the earthquake waves show us."
The study was published in journal Geophysical Research Letters.
