What is there in the center of the earth?

Seb, given all that pressure and all that insulation I'd long assumed it's fluid

I suspect you will see a sign telling you how many more miles to China

Another one of your baseless assumptions

Well then Tim,maybe at least very hot

Look up Adam Dziewonski and the PREM model or go to one of the Calif Uniz for a newer model called a REM model. These are tools that, like weather models, disagree and one guy takes shots at the other. Dzieonski's has been in use since 1980 and its pretty good for showing up the approximate different seismic zones beneath the earth.
There are at least 2 liquidus layers in the mantle and the core. Several mdels want to show an aditionl liquidus but , what the hell, nobody's yet drilled out any of them.

Penn State made up a nice cross sectional color 2d models based on PREM . Let it be said that the earth is spherically layered like a Smith Island Cake, lotsa layers , some wet. The central portion of the core is...... well that's another story.

This lady lives at the center of the earth wth TARZAN. She hot enuff?

this Wikipedia piece,Formation Of Planets, is worth a read
on the accretion theory of the solar system's origin.

If Earth started as a fragment of a solar nebula, then
it's conceivable that Earth still retains its some of its
original heat at the core.