What do you think you'd find if you dug a tunnel into the ground four feet deep? How about four MILES deep. How about four HUNDRED miles deep?

We'll you'd better keep digging, because to get to the very center of our planet, you'd have to dig down almost four THOUSAND miles! And you'd go through some very interesting sights on the way down–if you like dense layers of rock and molten magma that is!

Instead of going through all that trouble, let's just start right at the middle.