Researchers say Earth’s inner core shifted from moving slightly faster than the mantle to moving more slowly — a subtle change with possible connections to variations in day length and the planet’s deep magnetic machinery.

Something unusual has been happening more than 3,000 miles beneath our feet: Earth’s solid inner core has been retracing its position relative to the planet’s mantle.

The finding comes from a 2024 study published in Nature, whose researchers analyzed seismic data from 121 repeating earthquakes near the South Sandwich Islands between 1991 and 2023. According to the study, matching seismic waveforms indicate that the inner core gradually “super-rotated” from 2003 to 2008 before moving back along the same relative path from 2008 through 2023 at roughly two to three times the slower rate.

But “backtracking” does not mean the core suddenly started rotating in the opposite direction.

As the University of Southern California explained when announcing the findings, the inner core began moving more slowly than Earth’s surface around 2010. In other words, both continued rotating, but the inner core shifted from moving slightly ahead of the mantle to falling behind it.

Scientists cannot directly observe the inner core, so researchers rely on earthquake waves that travel through Earth’s interior. Repeating earthquakes are especially useful because waves originating from nearly the same location can provide something resembling a natural before-and-after comparison.

The finding could also offer clues to subtle variations in Earth’s rotation. USC geophysicist John Vidale said any resulting change in the length of a day would be extremely small — on the order of one-thousandth of a second — and difficult to distinguish from effects produced by the oceans and atmosphere. The 2024 study did not, however, establish that the inner core’s backtracking directly caused a measurable change in day length.

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The magnetic implications require similar caution. Earth’s primary magnetic field is generated by the movement of electrically conducting material in the liquid outer core, according to the U.S. Geological Survey. The solid inner core interacts with this larger system, but researchers have not shown that its recent backtracking caused a specific change in Earth’s magnetic field or anything resembling a magnetic-pole reversal.

And the picture has become more complicated.

A 2025 Nature Geoscience study led by Vidale found evidence that some seismic changes may result from deformation near the inner-core boundary rather than rotation alone. The researchers concluded that both processes may be involved.

What scientists are observing, then, is not a simple reversal but evidence that Earth’s deepest regions are dynamic on surprisingly short timescales. Precisely what drives those changes — and whether they follow a predictable long-term cycle — remains unresolved.

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