A mysterious earthquake deep below northern Utah had scientists scratching their heads back in 1979. The rumble seemingly occurred far lower beneath the Earth’s crust than scientists had believed was possible.

The tremor may not have been particularly strong, at a magnitude of 3.8, but the recorded seismic data threw experts for a loop nonetheless. The data suggested the rumbling had occurred over 55 miles below sea level, a depth that made no sense in conventional geology.

“I did some other analysis that convinced me of the reality of the deep depth but it was hard to convince others of the highly anomalous mantle earthquake occurring in a region where none should exist,” said George Zandt, who was a University of Utah seismology researcher at the time and helped record the unusual quake, in a new statement.

Now, as detailed in a study published earlier this year in the journal The Seismic Record, University of Utah geology professor Keith Koper and Zandt — who came out of retirement for the new investigation — analyzed eight subsequent “deep earthquakes” in the region, confirming they occurred in the Earth’s upper mantle, dozens of miles below the boundary of the crust.

Koper and his colleagues say they’ve determined that the quakes are an “archetypal continental mantle event,” meaning they’re related to movements in the Earth’s mantle that take place over extremely long time scales.

The research highlights how much there’s still to learn about these forceful tectonic dynamics deep inside the planet, and how surprisingly different they are from more shallow, crust-based seismic events.

“It’s sort of a mystery in terms of fundamental physics,” Koper said in a statement. “How in the world can these things happen?”

“Another reason why it’s a big deal is that we have no idea how big they can be,” he added. “With crustal earthquakes, we can measure what we think their maximum size is going to be. We measure the faults that we can map out near the surface.”

Unlike earthquakes that occur in the Earth’s curst, deep earthquakes don’t announce themselves through foreshocks and aftershocks. The team determined they occur at the western edge of the Wyoming Craton — a leftover block of our planet’s lithosphere, the rigid outermost shell of the Earth, which stretches across northern Utah and southwest Wyoming — where temperatures can exceed 1,300 degrees Fahrenheit.

The team suspects these new “deep quakes” could be caused by the mantle slowly squeezing by the Wyoming Craton.

“On the scale of millions of years, the mantle is hitting the craton and then flowing around it,” Koper explained. “It’s that interaction where that mantle flow is being diverted around this hard cratonic root that’s causing the increased strain rate, the increased deformation and it’s also creating extra stresses.”

“We think it’s that interaction between the keel of the iceberg and the medium around it that’s leading to these earthquakes,” he added.

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