Scientists are very curious to know the answer to this one too! Unfortunately, we don't know exactly what's down there because we can't travel to the center to look. (Why not?) We do, however, have some knowledge of what's inside the earth by looking at the composition of volcanic rocks (which were once very deep in the earth), meteorites (which may represent fragments of other planets), and the seismic waves generated by earthquakes. (A good project would be to learn how geologists can use earthquakes to learn more about the inside of the earth.) It seems that the core of the earth is made mainly of metal mostly iron, but also nickel, sulfur, and silicon. The center of the core is solid, but the outside part appears to be molten. Even more amazingly, scientists now think that the solid part of the core is slowly rotating beneath us! The core is surrounded by the mantle, which is a zone of dense rocky matter (lots of silica). On top of the mantle lies the crust, which is the part of the earth we're familiar with. The entire earth has a radius of about 6400 km, and the core has a radius of about 2900 km. The crust is only about 50 km thick!