For the first time, scientists have found what appears to be an extra layer of plate tectonics lurking in Earth's mantle under east Asia, and it could finally explain a mysterious series of earthquakes between Fiji and Australia.

Evidence of a huge slab of ancient Earth has been imaged under the tiny Pacific island of Tonga - right where almost all of the planet's deep earthquakes occur - and it's estimated to be travelling almost as fast as the tectonic plates at the surface.

"Basically, 90 percent of Earth's deep seismicity (more than 500 km deep) occurs at the Tonga area where we've found our long, flat slab," geologist Jonny Wu of the University of Houston told The Guardian. 

Earth's mantle is a huge layer of solid rock about 3,000 km thick, that moves like a really thick liquid under the crust, where we live. 

When the mantle moves, the crust moves along with it, and that's what shifts tectonic plates around.

Sometimes these plates are dragged apart, causing the ocean floor to be torn open and flooded with magma bubbling up from the mantle. But the opposite can happen too - plates slam into each other to cause earthquakes, mountain ranges, trenches, and volcanoes.

When plates collide, they can also prompt a process called subduction, which causes one of the two plates to be pushed down into the liquid mantle below, where it continues to sink down towards Earth's core.

Now, thanks to a new imaging technique that allows us to visualise Earth's inner structure using earthquake data, Wu and his team say they've discovered what appears to be multiple tectonic plates that were subducted 50 to 60 million years ago into the mantle.

They found them sitting in the transition zone - part of the mantle that sits between the upper and lower sections, at a depth 410 and 660 km (250 to 400 miles).

The find, which was announced at a joint conference of the Japan Geoscience Union and the American Geophysical Union in Tokyo this week, is yet to be independently confirmed, but if it is, some pretty hefty questions will follow. 

Perhaps the biggest question pertains to our understanding of Earth's crust and mantle, and the neat little boundaries we've given them.

"One underlying principle of plate tectonics is that of isostasy, which basically says that … you can, in broad lines, delineate an area as being 'the crust', since most plates will bob around this mean elevation, and there's no free magma on top, and 'the mantle', which is underneath this crust," Alexandru Micu explains for ZME Science.

But instead of being pushed down into the mantle to one day be recycled as raw crust material, these plates are behaving just as they would on the surface.

"Beyond their choice of neighbourhood, these sunken plates don't differ that much from traditional plates in behaviour," says Micu.

"They slide horizontally at about the same speeds as surface plates, and can travel thousands of kilometres from the point of subduction. They can bend the same way surface plates do, and the energy released during a break can generate earthquakes - again, pretty typical plate mannerisms."

The good news is they could also answer some burning geological questions - namely, what's been causing a series of deep, violent earthquakes, known as the Vityaz Earthquakes, which have been traced back to the mantle between Fiji and Australia.

Just like plates sliding in the crust lead to regular earthquakes, it could be that plates sliding in the mantle give rise to even deeper earthquakes, the team suggests.

To be clear, the research is only preliminary, and needs to be written up for peer-review before we can read too much into it.

But if one thing's clear, it's that our planet is hiding a whole lot on and below the surface that we still don't know about - like the possible new continent called Zealandia, or this ancient buried sea.

And now we could be seeing the first real signs that Earth's mantle and crust are far more complicated than we've given them credit for.

"This evidence of the subducted part of a plate maintaining some kind of integrity for more than a thousand kilometres as it is forced along the transition zone is intriguing," geologist David Rothery from the Open University in the UK, who wasn't involved in the study, told The Guardian.

"It will make us reassess the conventional idea of subducting plates being assimilated into the deeper mantle as they go down."

A paper outlining the team's earlier work can be accessed here.