Layers of the Earth

The Earth is made of layers. The crust is the thin, rocky outer layer we live on, between about 5 km thick under oceans and 70 km under mountains. Below it is the mantle, a very thick layer of hot rock that is solid but can flow very slowly, like thick toffee. At the centre is the core, made mostly of iron and nickel. The outer core is liquid and the inner core is solid because of the huge pressure.

Nobody has drilled through the crust, so how do we know? Scientists study how earthquake waves travel through the Earth. Some waves cannot pass through liquids, and they disappear on the far side of the Earth: this shows the outer core is liquid. The deeper you go, the hotter it gets: the core is about as hot as the surface of the Sun.