The deepest anyone has drilled into the earth is around 12 kilometres, we've only scratched the surface. This tells us that the inside must be made of something much denser than rock. A meteorite has roughly the same density as the whole earth. Iron and Nickel are both dense and magnetic.
Scientists can follow the path of seismic waves from earthquakes as they travel through the Earth.
We know that the mantle rocks are under extreme pressure, diamond is made from carbon deposits and is created in rocks that come from depths of 150-300 kilometres that have been squeezed under massive pressures. As we go deeper and deeper into the earth the temperature and pressure rises. This does not sound very thin but if you were to imagine the Earth as a football, the crust would be about millimetre thick. The continental crust in the Himalayas is some 75 kilometres deep. The mantle is the layer beneath the crust which extends about half way to the centre.