The core of the Earth is basically iron, at a temperature of ~6000 degrees and at a very high pressure.
A recent measurement of what iron looks like at those conditions suggests that its a hexagonal crystal, and that iron will only melt at even higher temperatures – so its probably not a liquid ball at all!
For reference: (Science 15 October 2010: Vol. 330 no. 6002 pp. 359-361 DOI: 10.1126/science.1194662)