Earth's inner core is a solid, rigid ball of iron

What actually happened

The evidence, as with most deep-Earth science, comes from earthquakes. Researchers compared seismic waves from 121 'repeating' earthquakes near the South Sandwich Islands, quakes from the same spot decades apart, recorded at stations in Alaska and Canada between 1991 and 2024. Waves that pass through the same part of the inner core should look identical if nothing changed; some of them did not, and the differences pointed to the core's surface physically shifting.

The interpretation is that the outermost layer of the inner core is not perfectly solid and static but can undergo slow viscous deformation, more like extremely stiff tar than steel, as the churning liquid outer core drags and pushes on its boundary. This came on top of separate 2024 work showing the inner core's rotation has slowed relative to the mantle, so the deep Earth turns out to be doing two dynamic things at once.

The update is to a mental image, the still, solid heart of the planet, that generations were taught as settled. The inner core is still solid in the bulk sense, but its surface is measurably changing shape on human timescales, which nobody expected, and it matters because that same iron ball helps generate the magnetic field and subtly influences the length of a day.