The surface of the Earth is a bumpy place, with terrain made up of smooth ocean floors to jagged mountain peaks and everything in between. But according to a new study, that's nothing compared to the landscapes found deep within the planet. Using data from one of the biggest earthquakes on record, geophysicists have now found massive mountain ranges hundreds of kilometers beneath our feet.

The researchers, hailing from Princeton University and the Institute of Geodesy and Geophysics in China, focused on a relatively-little studied region inside the Earth known as the transition zone. This narrow band, found between 410 and 660 km (255 and 410 mi) deep, separates the upper and lower mantle.

To get a closer look at what's going on down there, geologists study how seismic waves from huge earthquakes travel through and bounce off different materials. The biggest quakes can actually be strong enough to send those shockwaves rippling all the way through the core to the other side of the planet, and back again. So the team looked at data from the second-biggest deep earthquake on record – a magnitude 8.2 event that struck Bolivia in 1994.

The data showed the topography at the top and bottom of the transition zone. The top boundary, about 410 km down, was found to be relatively smooth. But at a depth of 660 km the lower boundary was surprisingly jagged, with elevation changes as drastic as 3.2 km (2 mi). The team says this makes it rougher than the surface of the Earth, but there are also fairly smooth sections.

The researchers say the discovery could help answer a few questions about inner Earth physics. It's unclear how much (if at all) the upper and lower mantle, on either side of the transition zone, mix together. The team says the smooth sections could be where the two parts of the mantle are mixing well, while the more jagged areas indicate the opposite.

So, how could this boundary have formed in the first place, and how has it persisted for so long? The researchers suggest that the remnants of ancient tectonic plates could be resting down there, after being pushed deep into the Earth through subduction zones like the Mariana Trench. The chemical differences introduced by these old rocks from the crust could create the jagged landscape.

"It's easy to assume, given we can only detect seismic waves traveling through the Earth in its current state, that seismologists can't help understand how Earth's interior has changed over the past 4.5 billion years," says Jessica Irving, an author of the study. "What's exciting about these results is that they give us new information to understand the fate of ancient tectonic plates which have descended into the mantle, and where ancient mantle material might still reside."

The research was published in the journal Science.