Vibrations caused by earthquakes suggest that the Earth's inner core is solid but viscous

With more than 5,000 km of rock between us and the Earth's core, it's hard to see clearly what's going on there. It has long been thought that the inner core is a solid iron ball, but it is difficult to find direct evidence. Researchers at the Australian National University (ANU) have detected a type of seismic wave crossing the nucleus that is propagated only by solid objects.