Firm evidence that Earth’s core is solid

Faint yet distinct ground motions recorded by a large network of seismic instruments in Japan in early 2006 are the strongest, most direct evidence that Earth’s inner core is solid.

On February 22, 2006, a magnitude-7 quake rocked Mozambique. The temblor was an unusually large one for southern Africa, but it also was quick for its size: Motions at the epicenter lasted only eight seconds or so, says George Helffrich, a geophysicist at the University of Bristol in England.