New research published in the journal Nature Geoscience analysed seismic waves from repeating earthquakes over the last six decades.
Research says that Earth's solid inner core, a hot iron ball, has stopped spinning faster than the planet's surface and might now be rotating slower than it.
The Earth's core is roughly 5,000 kilometres (3,100 miles) below the surface we live on, this "planet within the planet" can spin independently because it floats in the liquid metal outer core, an AFP report said.
Exactly how the inner core rotates has been a matter of debate between scientists - and the latest research is expected to prove controversial.
What little we know about the inner core comes from measuring the tiny differences in seismic waves - created by earthquakes or sometimes nuclear explosions - as they pass through the middle of the Earth.