The first thing to remember is that NOBODY has ever been there, so what you are about to hear is barely past the Wild and Crazy Idea stage. What we think we know comes from a study of how earthquake (seismic) waves travel through the earth, and how long it takes for them to get from where the earthquake happens to a recording station. 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.