Forty-eight hundred kilometres beneath the surface, a ball of iron and nickel roughly the size of the Moon is doing something scientists have debated for thirty years: it is changing its mind about which way to spin. Two landmark studies – one published in Nature Geoscience in January 2023, the other in Nature in June 2024 – have now converged on a single, unambiguous conclusion. Earth’s solid inner core, which for decades was thought to rotate slightly faster than the planet’s surface, has not merely slowed. It has reversed direction relative to the mantle, and it is now lagging behind. A third paper, published in February 2025, adds a startling coda: the core’s surface is not just turning differently, it is changing shape.
This is not speculative science. It is the product of painstaking analysis of seismic waves from earthquakes in the South Sandwich Islands – 121 of them, recorded between 1991 and 2023 – and it carries implications for everything from the length of our days to the behaviour of the magnetic field that shields us from solar radiation.
The First Clue: A Seven-Decade Swing
The story begins with Yi Yang and Xiaodong Song of Peking University, who published their findings in Nature Geoscience in January 2023. Their team analysed seismic waves from repeating earthquakes – quakes that occur at the same location years apart, producing nearly identical seismograms. By comparing these waveforms, the researchers could detect whether the inner core had moved relative to the mantle between measurements.
What they found was striking. All the seismic paths that had previously shown significant temporal changes had exhibited little change over the past decade. This globally consistent pattern suggested that the inner core’s differential rotation had recently paused. Going further back, using Alaskan seismic records of South Sandwich Islands doublets going back to 1964, Yang and Song identified a turning point in the early 1970s, followed by a gradual turning-back of the inner core as part of an approximately seven-decade oscillation.
“We believe the inner core rotates, relative to the Earth’s surface, back and forth, like a swing,” Yang and Song told AFP. “One cycle of the swing is about seven decades.”
The inner core began rotating slightly faster than the rest of the planet in the early 1970s, then slowed before coming into sync with Earth’s rotation around 2009. Since then, a “negative trend” has taken hold – the core now rotates slower than the surface. The researchers predicted the next turning point would occur in the mid-2040s.
Confirmed: The Core Is Backtracking
The Peking University findings were provocative but controversial. Seismologists have long compared their profession to doctors examining patients with imperfect equipment – “like studying internal organs without a CT scan,” as John Vidale of the University of Southern California put it. Many in the field urged caution.
Then, in June 2024, Vidale’s team delivered what they described as the most convincing resolution yet. Published in Nature, their study analysed the same pool of 121 repeating earthquakes from the South Sandwich Islands, but employed a different methodology – focusing on waveform change reversals rather than arrival-time differences. The results were unambiguous: the inner core had begun decreasing its speed around 2008, and by 2010 was moving slower than the Earth’s surface for the first time in approximately four decades.
“When I first saw the seismograms that hinted at this change, I was stumped,” Vidale said. “But when we found two dozen more observations signaling the same pattern, the result was inescapable. The inner core had slowed down for the first time in many decades.”
The USC study also incorporated data from twin Soviet nuclear tests between 1971 and 1974, as well as repeated French and American nuclear tests from other studies, extending the observational record further into the past. Vidale attributed the slowdown to two forces: the churning of the liquid iron outer core, which generates Earth’s magnetic field, and gravitational tugs from dense regions of the overlying rocky mantle.
The implications for the length of day, Vidale noted, are vanishingly small – on the order of a thousandth of a second, “almost lost in the noise of the churning oceans and atmosphere.”
Not Just Slowing – Shape-Shifting
If the 2023 and 2024 studies settled the question of rotation, a paper published in Nature Geoscience in February 2025 opened a new and unexpected chapter. Vidale and his colleagues extended their analysis of the South Sandwich Islands earthquakes, this time comparing pairs at times when the inner core had re-occupied the same rotational position. They expected to find identical waveforms. Instead, they discovered subtle but persistent differences – particularly in waves that had grazed the outermost surface of the inner core rather than penetrating deep into its interior.
The conclusion: the inner core’s surface is deforming. Flow in the liquid outer core appears to be stirring up the outermost layer of the solid inner core, altering its topography. “The most likely thing is the flow in the outer core is stirring up the outermost inner core a little bit and changing the topography,” Vidale told Live Science. He estimated the deformation could involve vertical shifts of hundreds of metres, possibly a kilometre or two, spread across regions hundreds of kilometres wide.
This finding resolves what had been a persistent puzzle. Since the first studies of the inner core in the 1990s found evidence of movement, there had been debate over whether temporal variability in seismic waves resulted from rotation, or from more localised changes near the inner-core boundary. Vidale’s 2025 paper suggests the answer is tentatively both.
Why It Matters: The Planetary Dynamo
The inner core sits at the heart of Earth’s magnetic engine. Though it does not itself generate the geomagnetic field – that is the province of convective currents in the liquid outer core – its solid structure stabilises those currents. The inner core is growing by roughly a millimetre per year as liquid iron from the outer core crystallises onto its surface, a process that drives heat movement in the outer core and, by extension, the geodynamo.
Bruce Buffett, a geophysicist at the University of California, Berkeley, who was not involved in the 2025 study, told Live Science that understanding the inner core’s texture, structure and dynamics could illuminate the history of Earth’s deepest processes. “The hope is we might be able to use the texture of the inner core and its structure and dynamics to say something about the history of the dynamics of the deepest part of the planet,” he said.
The magnetic field itself is not immediately threatened. Earth has experienced magnetic excursions before – most notably the Laschamps event approximately 42,000 years ago, when the field weakened dramatically and partially reversed for roughly a thousand years. That event correlates with increased cosmic radiation, shifts in climate patterns, and possible stresses on evolutionary lineages. But researchers caution that any connection between the inner core’s current behaviour and future magnetic disruptions remains speculative.
The Scientific Debate Is Far From Over
Not all experts are convinced the oscillation model captures the full picture. Some researchers have proposed that the inner core’s rotation rate changes on shorter timescales – every 20 to 30 years rather than seven decades – and that the relationship between core rotation and magnetic field reversals is more complex than a simple cycle suggests.
Song himself, while acknowledging the strength of the evidence, has urged continued caution. “We’ll need to keep accumulating the data and keep searching for the inner core behaviours,” he told Live Science. “I won’t be surprised by future surprises about the inner core behaviours as we keep searching.”
Yi Yang, Song’s co-author now at Nanjing University, called Vidale’s shape-changing interpretation plausible but noted the data remains limited to a fraction of the outermost layer. The field, in other words, is advancing rapidly – but it is advancing into the unknown.
Instruments of Discovery
The technical achievements underpinning these studies deserve attention. The International Seismological Centre’s global network monitors thousands of seismic events annually, but the specific technique used in the inner-core studies relies on “repeating earthquakes” – quakes that originate at virtually the same location and produce near-identical waveforms when recorded at the same stations years apart. The South Sandwich Islands, a volcanic arc in the South Atlantic, provide an unusually rich source of such events.
Meanwhile, the European Space Agency’s SWARM satellite constellation tracks real-time shifts in the geomagnetic field from orbit, offering a complementary perspective on the deep processes unfolding below. Machine-learning models are now being trained to decode seismic noise at higher resolution, potentially offering sharper insights into core dynamics in the coming years.
A Planet in Quiet Flux
None of this will alter daily life. The millisecond shifts in day length caused by the core’s deceleration are detectable only by atomic clocks. The magnetic field shows no sign of imminent collapse. The tectonic plates will not suddenly lurch.
But the research carries a deeper significance. Earth’s inner core is, in a sense, the planet’s most fundamental clock – a feature of planetary structure that has operated out of sight and out of mind since the core first solidified, somewhere between one and one and a half billion years ago. That it oscillates, that it deforms, that it responds to forces both electromagnetic and gravitational, tells us something about the nature of the world beneath our feet: it is not static. It is not inert. It is a system in motion, governed by laws we are only beginning to observe.
As Vidale reflected: “The dance of the inner core might be even more lively than we know so far.”
The mid-2040s – when Song and Yang predict the next turning point – are not distant. By then, a new generation of seismologists will have accumulated decades of additional data, and the models will have been tested against reality once more. Earth’s hidden heart may yet reveal secrets that reshape not only our understanding of planetary physics, but our conception of the ground we stand on.
