The scientists estimate the precious minerals are buried more than 100 miles below the surface, far deeper than any drilling expedition has ever reached.
Dr Ulrich Faul, a research scientist at MIT, explained that one of diamond's many special properties is the speed at which sound waves travel through the material.
When the waves moved through the roots of the ancient cratons, the researchers noticed that they sped up considerably more than expected.
Sound waves travel at different speeds depending on the composition, temperature, and density of the rocks and minerals they travel through, giving scientists a method to estimate what types of rocks are found below the Earth's surface by comparing the velocities of these sound waves, according to MIT News.
An global team of researchers has used sound waves to estimate the composition of the Earth's upper mantle and they think that about a quadrillion tons of diamonds are buried somewhere over 160 kilometers (100 miles) deep beneath our feet. In studying how fast or slow these waves (triggered by earthquakes, tsunamis, explosions, and other ground-shaking sources) move through the Earth, geologists are able to figure out what type of rock the waves are passing through.
The researchers estimate that the roots, or bottom sections, of these cratons may be composed of 1-2% diamond.
After conducting a series of experiments in the lab where they sent sound waves through different rocks, the researchers found that only rock containing 1-2% diamond, among other components, could produce the velocities recorded in the craton roots.
Scientists can also use this seismic data to construct an image of what the Earth's interior might look like.
The thing is that we can't actually get to them, but they're there, and they're a lot, more than we could ever imagine.
Cratons are stable parts in the crust and mantle of the Earth that are usually less dense and colder than the parts surrounding them. The number they came up with: 1016 tons of diamond, or over 1,000 times more than previously thought.
Faul and his colleagues calculated that the anomaly could be caused by 1%-2% of diamonds in the 'cratonic roots'. This combination would make cratons less dense than previously thought, which would explain why the seismic waves travel faster than older models called for.
"Diamond in many ways is special", Faul says.
"They are like pieces of wood, floating on water", Dr. Faul said. "Cratons are a tiny bit less dense than their surroundings, so they don't get subducted back into the Earth but stay floating on the surface".
Where are diamonds found?
"The velocities that are measured are faster than what we think we can reproduce with reasonable assumptions about what is there", Faul says.