The basic idea is that different materials transmit seismic waves at different speeds. With a lot of earthquakes and a lot of recording stations, geophysicists are beginning to get a pretty detailed picture of what is probably down there. One of the most distinctive features of the earth's interior is how it seems to be layered by density , with the heaviest stuff in the center, and the lightest material at the surface. In fact, the earth probably looks a lot like a hard boiled egg if you could cut it open.
Most geophysicists think that the core is composed of high density materials like iron and nickel.
The egg's shell is like the earth's crust - a thin veneer of rigid, low density material at the surface. And all the white stuff in between is like the earth's mantle - the largest layer which, in the case of the earth, is of medium density, and, in the case of an egg, tastes best with a bit of salt and pepper. Thanks to the Univ. The crust is REAL thin relative to the size of the earth - much, much thinner than an eggshell, and is of much lower density than the core. It is probable that the mantle represents the vast majority of the earth's mass which is still trying to figure out if it is heavy enough to be accepted at the core, or is lower in density and therefore has to float about on the surface with the rest of the scum.