Earth’s inner core may have quietly reversed course: after decades of spinning slightly faster than the surface, seismic waves from repeating earthquakes show it slowed around 2010 and began drifting backward relative to the rest of the planet.

More than 5,000 kilometres beneath your feet, Earth’s solid inner core has spent the past few decades turning slightly faster than the planet wrapped around it. Seismic measurements suggest that around 2010 it slowed to match the surface, and has since fallen behind, drifting backward relative to the rest of the Earth.

It is a strange idea, and an easy one to overstate. The core has not stopped, and it has not begun spinning the other way. What has changed is its speed relative to everything above it.

How you measure something you cannot see

The inner core is a ball of iron and nickel about the size of the Moon, sitting at the centre of the Earth under crushing pressure. No one can observe it directly. Almost everything known about its motion comes from the one thing that reaches it and returns: the seismic waves of earthquakes.

The method relies on repeating earthquakes, quakes that strike in nearly the same spot years apart and send almost identical waves along the same path through the core. If the core has rotated between one quake and the next, those waves arrive subtly changed. Tracking that drift across decades, with help from the records of old underground nuclear tests, is how the core’s rotation is read.

What the data show

In 2023, Yi Yang and Xiaodong Song at Peking University reported in Nature Geoscience that the inner core had been super-rotating, turning a little faster than the surface, since around the 1970s, then slowing through the 2000s until it came into step with the planet about 2009. Since then the trend has been negative. The core is now turning slightly slower than the surface, which is what “drifting backward” means here.

A separate team at the University of Southern California reached much the same conclusion the following year. The picture both groups favour is not a single reversal but an oscillation, the core swinging back and forth relative to the surface, with one full cycle lasting roughly seventy years.

What “reversed” does and does not mean

This is the part worth slowing down on, because the headlines rarely do.

The motion is relative.

Set against the stars, the inner core still spins the same way the whole Earth does, once a day. What shifted is the small difference between the core’s spin and the mantle and crust above it, from a touch faster to a touch slower. The differences are tiny, fractions of a degree a year, and at the surface they show up only as changes of a few thousandths of a second in the length of the day. Nothing lurches, and no one feels it.

Why it is still argued over

The evidence is indirect and thin. A handful of repeating-earthquake paths, read across half a century, is a slender basis on which to claim a seventy-year cycle, and not everyone reads the same records the same way. Some recent work suggests that part of what looks like a change in rotation may instead be the inner core’s surface slowly changing shape, which would alter the seismic waves in a similar manner.

So the narrower claim is fairly solid: the core’s motion relative to the surface slowed and turned around 2010. The fuller story, the oscillation and whatever drives it, is still being worked out.

Why it matters

The reason geophysicists care is that the inner core does not move in isolation. The same roughly seventy-year rhythm appears in two other places: in tiny variations in the length of the day, and in the slow drift of Earth’s magnetic field. That overlap hints that the core, the mantle and the churning molten iron that generates the planet’s magnetism are linked, trading angular momentum back and forth.

The next test is simply time. Watching how the rotation behaves over the coming decade, and reconciling it with the evidence that the core is also changing shape, is what will show whether the oscillation model holds.