Sir Isaac Newton was one of the first scientists to calculate that the average density of the Earth must be twice that of the surface rocks and therefore proposed the existence of a dense interior. We now know that the Earth's core is composed principally of iron and consists of a solid, crystalline inner core inside a liquid outer core. The inner core started to develop as the Earth began cooling after its initial formation, and it continues to grow: under the prevalent extreme conditions of both temperature and pressure, the dense iron crystallizes into solid form out of a mush of other elements.

The inner core exhibits some puzzling qualities. Separated from the rest of Earth by the fluid outer core, it can wobble and rotate. Seismic waves that are generated by earthquakes and propagate through the inner core travel much faster when going from south to north than when travelling in a direction parallel to the Equator. The inner core also exhibits an east–west asymmetry, with seismic waves travelling slightly faster in the upper surface of the eastern hemisphere compared with that of the western hemisphere. Furthermore, seismic waves are observed to travel slowly through a thick layer that surrounds the inner core. This layer is thought to be a dense fluid.

Thierry Alboussière at the Joseph Fourier University, Grenoble, and colleagues present a theory to explain these curiosities (Nature 466, 744–747; 2010). They propose that instead of sitting at the precise centre of the Earth, the inner core could be offset to the east: if the density of the solid inner core was higher, say, in the west, the entire mass would be pushed slightly eastwards by the forces of gravity. If so, the western side of the inner core experiences greater pressure compared with the east and hence the iron crystallizes faster. Similarly, on the eastern side the iron melts.

The proposed process is self-reinforcing: as the inner core grows in size, the newly solidifying iron in the west crystallizes at increasingly low pressures and is thus cooler and denser than the liquid melt in the east, so the inner core is kept in its offset position. Over time, iron solidified at the western boundary migrates eastwards. And most of it — all that does not contribute to the net growth of the inner core — finally melts back into the outer core.