For about half a century now, Utah has been getting hit with a series of earthquakes deep below its surface. Seismologists studying the phenomenon couldn’t explain them. No one thought an earthquake could happen so far beneath Earth’s crust. According to new research from the University of Utah and published in The Seismic Record, scientists now believe they’re just starting to understand what’s going on.

The mystery began on February 24, 1979, when instruments at the University of Utah detected a magnitude 3.8 earthquake beneath the tiny town of Randolph. Almost nobody felt it. Seismologist George Zandt analyzed the data. All his research was telling him that the quake started around 90 kilometers below the Earth’s surface; that’s deep below the crust and deep inside the Earth’s mantle. Back then, it seemed like an impossible answer, so ridiculous that it had to have been wrong. Earthquakes just don’t happen there.

All these decades later, they’re starting to realize that maybe they do.

Scientists Still Have a Lot of Questions About These Earthquakes

A new team of researchers went back to the old data and compared it to several similar earthquakes that were recorded across Utah and southwest Wyoming. They found that at least nine earthquakes originated deep within the mantle, creating a new category of earthquakes called “continental mantle earthquakes.”

These are quakes that behave unlike any other type of earthquake. They have no foreshocks or aftershocks. It’s just the quake and then nothing, and all happening in a part of the Earth where temperatures can exceed 700 degrees Celsius, meaning rock is usually so flexible, so invaluable, that it doesn’t quake and rattle apart. Yet, that’s exactly what’s happening; it still fractures enough to spark an earthquake.

The researchers know what’s happening more or less, but the why of it is still a mystery. They might have an answer, and it would be a huge block of old lithosphere (the uppermost part of the Earth’s crust, the one closest to the bottom of our feet) in the Wyoming Craton that stretches deep into the Earth’s crust. The research team compares that errant chunk of lithosphere to the keel of a boat, the long fin along the underside of a seafaring vessel that keeps it stable. They think that over millions of years, stress builds at the interaction point between the flowing material in the Earth’s mantle and the lithosphere’s “keel,” eventually becoming strong enough to trigger earthquakes deep below the Earth’s surface.

Even if that is the explanation, the researchers still don’t understand the physics of it all and have no way to measure how big one of these continental mantle earthquakes could be. The answer only raises more questions, but at least it’s a start.