The Cross-Circle of the Earth

The cross-section of the Earth looks a bit like an onion, so if you take an onion apart, it has layers of different material. So, just the top 30 kilometres has all the rocks which you're used to see, and we call that the crust. And then, when you go down underneath that, you get different stuff, and that different stuff is pretty much the same all the way for the next 3,000 kilometres down. Then there's a really big surprise, because in the centre of the Earth, there's a ball, and that ball is about 3,000km in radius. It sits inside the Earth, and here's the big surprise, is that ball