Earth's inner core rests 3,200 miles below the crust, separated from the mantle by the liquid outer core which allows the inner core to rotate at a different speed from the planet's rotation. Researchers studied seismic waves from repeating earthquakes that have passed through the Earth’s inner core over the last six decades to infer how fast the inner core is spinning, Xiaodong Song and Yi Yang of China's Peking University published Monday in the journal Nature Geoscience. The study's authors found that the inner core's rotation "came to near halt around 2009 and then turned in an opposite direction." Since then, seismic records –which previously changed over time, showed little difference. Visit the link in bio to learn more.