Buried more than 3,000 miles beneath our feet, Earth’s solid inner core was once thought to be unchanging—locked in place at the heart of the planet. But new research has revealed something much more dynamic. According to a recent study published in Nature Geoscience, Earth’s inner core has not only slowed down, it now appears to be rotating in reverse compared to the planet’s surface.
This discovery marks a turning point in our understanding of Earth’s deepest interior, confirming what scientists have long suspected but never been able to measure with such clarity: that the core is not static, but part of a much larger, shifting system that affects everything from magnetic fields to the length of our days.
The Planet’s Deepest Engine is Changing Course
Earth’s structure is layered like an onion—from the crust we walk on, down through the mantle, to the swirling liquid outer core, and finally to the solid iron inner core at the very center. That core, roughly the size of the Moon, is surrounded by molten metal and was long believed to rotate slightly faster than the rest of the planet.
But that’s no longer the case. Using decades of seismic data from earthquakes and even old nuclear tests, scientists have found “unambiguous evidence” that the inner core began slowing down around 2010—and has since started rotating backwards relative to Earth’s crust and mantle.
“When I first saw the seismograms that hinted at this change, I was stumped,” said John Vidale, lead author and professor of Earth sciences at USC. “But when we found two dozen more observations signaling the same pattern, the result was inescapable.”
Earthquakes and Nuclear Blasts Helped Detect the Shift
To track the core’s motion, researchers analyzed waves from 121 repeating earthquakes near the South Sandwich Islands, recorded between 1991 and 2023. They also reviewed signals from Soviet nuclear tests in the 1970s and American and French tests from the same era. These waves travel through the planet, reflecting off the inner core and revealing subtle changes in its position and behavior.
What they found wasn’t just a change in speed—it was a reversal. The seismic wave patterns indicated that Earth’s core, once rotating slightly faster than the surface, had slowed to a crawl and was now drifting in the opposite direction.
Vidale attributes this to powerful forces beneath the surface: “The churning of the liquid iron outer core that surrounds it, which generates Earth’s magnetic field, as well as gravitational tugs from the dense regions of the overlying rocky mantle.”
A Planet That Ticks Ever So Slightly Differently
This reversal likely has implications for day length, though the changes are so small—on the order of thousandths of a second—that they’re almost impossible to notice.
Still, they matter. Earth’s rotation speed, magnetic fluctuations, and possibly even tectonic activity may all be influenced by this deep geodynamic interaction between the solid inner core and the swirling outer core that envelops it.
“What we’re observing in this study for the first time is likely the outer core disturbing the inner core,” Vidale noted. The new findings suggest that the inner core doesn’t just spin like a rigid ball—it may adapt, deform, and even shift shape over time.
The Core’s Boundary May be Changing Too
Surprisingly, this research didn’t just point to a rotational reversal—it also showed hints of structural changes in the inner core’s boundary.
Waveforms from seismic stations in Alaska, Canada, and elsewhere suggest that the outer layers of the inner core may be slowly reshaping due to what scientists call viscous deformation. In other words, even the deepest part of our planet may be fluid and reactive on long timescales.
“As I was analyzing multiple decades’ worth of seismograms, one dataset of seismic waves curiously stood out from the rest,” Vidale said. “At first the dataset confounded me. But modern techniques have since sharpened the picture.”
Why It Matters Far Beyond the Core
While the idea of a spinning, reversing core may sound like science fiction, it plays a real role in the systems that make Earth habitable. The movement of molten metal around the inner core generates the planet’s magnetic field, which protects us from solar radiation and helps guide everything from animal migrations to satellites.
If the inner core is behaving differently than previously thought, it may help scientists better understand magnetic reversals, geologic cycles, and even how Earth’s internal processes compare to those of other planets.
This discovery doesn’t mean Earth is in danger—but it does mean our planet’s deepest engine is alive, complex, and still full of surprises.
