More than 5,000 kilometres beneath our feet, Earth contains a solid ball of iron-rich metal surrounded by a liquid outer core. No instrument can watch that inner core turn. Instead, researchers look for clues in seismic waves: the vibrations from earthquakes that travel through the planet and emerge at monitoring stations far away.
By comparing waves from earthquakes that happened repeatedly in nearly the same place, a team of researchers found evidence that the inner core’s rotation relative to Earth’s mantle slowed around 2010. Their 2024 study suggests the core may now be turning slightly more slowly than the mantle—or, in that relative frame, moving backwards. It does not mean the core has stopped, reversed its motion in an absolute sense, or changed the planet’s rotation in a way people could feel.
A hidden sphere, tracked through earthquakes
Earth’s inner core is solid because the pressure at the planet’s centre is immense, even though temperatures there are extremely high. Around it, the outer core is liquid. Above that lie the mantle and the thin crust at the surface. The inner core is inaccessible to direct observation, so scientists use seismic waves to investigate its structure and behaviour.
Some earthquake waves pass through the inner core. As the core’s position or physical properties change, the waves’ paths and travel times can change too. But interpreting those differences is difficult: an earthquake’s source, the rock along the wave’s path and the instruments recording it can all affect a seismogram.
The 2024 study addressed part of that problem by examining repeating earthquakes—events that occurred near one another and had similar seismic signatures. The researchers compared records of waves that crossed the inner core from these events, including earthquakes near the South Sandwich Islands recorded over several decades. They also analysed seismic signals from historical underground nuclear tests.
Because the events in a repeating sequence are so similar, differences between their recordings can help reveal changes along the waves’ route, rather than simply differences in the earthquakes themselves. The team found patterns in those signals that it interpreted as evidence of a change in the inner core’s motion relative to the mantle.
What “slowing down” means
Earth’s inner core is not a separate object spinning freely in empty space. It sits inside a planet whose layers interact through gravity and other forces. When scientists describe its rotation, they are talking about its motion relative to the mantle, the thick rocky layer above the outer core.
Earlier research had suggested that the inner core could rotate a little faster than the mantle. The new study’s interpretation is that this relative motion slowed, with the inner core no longer outpacing the mantle as it may have before. Calling that “backwards” can be misleading: it describes a comparison between two parts of a moving planet, not the core reversing direction like a wheel on a table.
The finding also does not say that Earth as a whole has slowed down, or that the planet’s magnetic field has suddenly changed. The magnetic field is generated mainly by the movement of electrically conducting liquid iron in the outer core. The solid inner core is part of the same deep system, but the 2024 study was about interpreting seismic evidence for rotation—not a forecast of magnetic-field behaviour.
A result with limits
Seismologists infer the inner core’s motion from indirect measurements, and researchers continue to debate how best to separate that motion from other changes within Earth. The study offers evidence for a slowdown, but it is not a direct video or a measurement of a visible object turning. Different datasets, methods and time periods can produce different interpretations of how the inner core moves.
That caution matters because popular accounts can make the result sound more dramatic than it is. There is no indication that the inner core has stopped, that the surface will suddenly reverse direction, or that people should expect a noticeable change in daily life. Any influence on the length of a day would be tiny, and the paper’s central finding is about relative motion deep inside Earth.
Instead, the work shows how much information can be extracted from earthquakes that repeat over time. A seismic station cannot see through thousands of kilometres of rock and metal, but it can record a wave that has crossed the inner core and detect subtle differences in its journey. Each new set of observations helps scientists test whether the core’s motion varies over time—and how that hidden movement fits into the larger workings of the planet.
For now, the most accurate takeaway is also the least sensational: evidence suggests Earth’s inner core has changed its rotation relative to the mantle. Understanding exactly how and why will require continued observation and comparison, using earthquakes as probes of a region no human instrument can reach.
