Researchers from the Massachusetts Institute of Technology (MIT) believe they have found a substantial cache of diamonds hidden in the Earth’s interior.

In a study, researchers report than more than a quadrillion tons of diamonds may be buried more than 100 miles below the surface, scattered within cratonic roots—the oldest and most immovable sections of rocks that lie beneath the center of most continental tectonic plates. The cratons–which can stretch as deep as 200 miles through the Earth’s crust and mantle– are shaped like inverted mountains.

“This shows that diamond is not perhaps this exotic mineral, but on the [geological] scale of things, it’s relatively common,” Ulrich Faul, a research scientist in MIT’s Department of Earth, Atmospheric, and Planetary Sciences, said in a statement. “We can’t get at them, but still, there is much more diamond there than we have ever thought before.”

The researchers made this discovery after noticing an anomaly in seismic data. Seismic data is regularly collected from sound waves that travel through the Earth that are triggered by earthquakes, tsunamis, explosions and other ground-shaking sources.

The data is generally used to construct an image of what the Earth’s interior may look like by estimating the types of rock that make up the crust and parts of the upper mantle.

However, scientists have been unable to explain why sound waves tend to speed up significantly when passing through the roots of ancient cratons, which are known to be colder and less dense than the surrounding mantle, theoretically yielding slightly faster sound waves.

“The velocities that are measured are faster than what we think we can reproduce with reasonable assumptions about what is there,” Faul said. “Then we have to say, ‘There is a problem.’ That’s how this project started.”

The researchers sought out to identify the composition of cratonic roots by creating a 3D model of the velocities of the seismic waves traveling through the Earth’s major cratons.

They then assembled virtual rocks from various combinations of minerals and calculated how fast sound waves would travel through each virtual rock. The only type of rock that produced the same velocities as what the seismologists measured, contained 1 to 2 percent diamond, along with peridotite and minor amounts of eclogite.

“Diamond in many ways is special,” Faul said. “One of its special properties is, the sound velocity in diamond is more than twice as fast as in the dominant mineral in upper mantle rocks, olivine.”

Diamonds are forged in high-pressure, high temperature environments of the deep Earth and only make it close to the surface through volcanic eruptions that occur more than 10 million years apart. The eruptions carve out geologic pipes made from kimberlite.