The inner core of the Earth is a dense ball of solid iron and nickel with a radius of about 1280 km that is surrounded by the outer core - a 2210-km thick layer of liquid metal. The next layer, the mantle, is made of molten rock and has a thickness of about 2850 km, while the topmost layer, the crust, is less than 100 km thick. It is thought that the Earth’s magnetic field is produced by the rotation of the outer core and mantle.

Just like electromagnetic waves, the seismic waves produced by earthquakes refract at boundaries between two different media. Therefore by detecting seismic waves that have been created on the other side of the globe, it is possible to extract information about the earth's interior.

Zhang, Song and co-workers compared the seismic waves from "Earthquake doublets" - pairs of quakes which occurred at the same location but at different times. They studied 18 doublets in which the waves were created by quakes in the South Sandwich Islands in the South Atlantic Ocean and detected at 58 seismic stations in and near Alaska. The largest gap between the two quakes in any doublet was 35 years. Each quake results in a characteristic signal known as a "waveform".

They found that when the waves did not pass through the inner core, both waveforms in the doublet were the same. However, if the waves had passed through the inner core and were more than four years apart, the waveforms were different. This means that something must have changed along the path followed by the waves during this period.

"This shows unambiguously that the inner core is moving relative to the mantle and crust," says Song. The next challenge for Song and other geophysicists is to develop a better model of the inner core and to explore its rotation is more detail.