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.