http://www.newsweek.com/scientists-finally-have-direct-proof-earths-core-solid-1178530

Researchers from the Australian National University are confident they have found direct proof, for the first time, that the earth’s inner core is solid—albeit a little bit squishy.

In a paper published in the journal Science, Hrvoje Tkalčić and Than-Son Phạm describe how they detected so-called "shear waves" or “J waves” in the inner core. These are a type of wave that can only travel through solid objects.