Geologists in the US say Earth might have a previously undiscovered layer of ultra-strong rock hiding in its mantle, some 1,500 km (930 miles) beneath our feet. 

The new layer is up to three times stronger than rocks in the less viscous upper mantle, and could explain why portions of sinking tectonic plates sometimes stall and thicken at this depth - a phenomenon that has for years puzzled geologists.

The finding challenges the existing understanding of Earth's internal structure, and researchers say, if it's true, it might also help explain the occurrence of earthquakes in the deep mantle.  

Earth's main layers include its relatively thin crust, which extends from just below the surface to about 80 km deep. Next there's the mantle, which extends to about 2,900 km deep, followed by its iron core. 

"Most layers are defined by the minerals that are present," said geophysicist Lowell Miyagi from the University of Utah in a press release. "Essentially, we have discovered a new layer in the Earth. This layer isn't defined by the minerals present, but by the strength of these minerals."

The team's results, which were published in the journal Nature Geoscience, suggest the new, ultra-stiff rock layer is located somewhere near the the middle of the mantle, and is temporarily trapping subducting plates. 

This is intriguing because the dominant mineral ingredients in the mantle, bridgmanite and ferropericlase, do not show any "structural transitions at these depths", the authors note.

Traditionally, "only moderate and smooth viscosity variations are expected with depth to at least around 2,500 km," they write.     

But they have found that the two minerals, under high enough pressure, can indeed transform and harden. 

The researchers crushed thousands of crystals of ferropericlase - with diameters thinner than a human hair - between two diamond anvils in a press. In doing so, they were able to simulate the pressures that act on these crystals at different depths in the mantle.  

As they squeezed the crystals, they bombarded them with X-rays from an accelerator to measure the distance between the atoms, which helped them determine the strength of the mineral at various pressures. 

They found that ferropericlase's strength starts to increase at pressures equivalent to those around 650 km deep, which is the boundary between the upper and lower mantle, and that the strength of the mineral increases threefold at pressures equivalent to those at 1,500 km. 

What's more, when the team simulated how ferropericlase behaved when mixed with the other dominant mineral, bridgmanite, they calculated that the strength at 1,500 km deep was 300 times greater than at the upper-lower mantle boundary.

"The result was exciting," Miyagi said in the press release. "This viscosity increase is likely to cause subducting slabs to get stuck - at least temporarily - at about 930 miles [1,500 km] underground." 

Tectonic plates, driven by the upward pressure of heat rising from Earth's core, glide along the top of the mantle. When oceanic and continental plates collide, the leading edge of the oceanic plate bends into a slab, which is forced underneath its counterpart, and descends into the mantle. This geologic process, known as subduction, causes volcanism and earthquakes.

These earthquakes mostly occur in the crust or very shallow mantle. But if something obstructs these sinking slabs, it could cause the slab to buckle or break, and might result in earthquakes happening much deeper in the mantle, says Miyagi

The presence of this rock layer, which is essentially a barrier, could also challenge conventional estimates about Earth's internal temperature.   

"If you decrease the ability of the rock in the mantle to mix, it's also harder for heat to get out of the Earth, which could mean Earth's interior is hotter than we think," he says

Miyagi calculates that the temperature around 1,500 km, where the layer would be strongest, is about 2,150 degrees Celsius - 600 degrees Celsius warmer than estimates for the upper-lower mantle boundary.

We're continually learning more about the inner-workings of planet. Scientists measuring the echoes produced by earthquakes recently suggested that Earth's inner core very likely had an inner core, comprised of different mineral structures.