Structure of the Earth

The ground beneath you feels solid and endless. Yet the deepest hole humans have ever drilled is only about 12 km deep, a tiny pinprick compared to the 6371 km down to the centre of the Earth. So what is down there? The Earth is built like a giant ball of nested shells, and we know them surprisingly well even though nobody has ever been there.

The Earth's shells

On the very outside lies the crust, the layer we live on. It is surprisingly thin: only about 7 km under the oceans and on average about 35 km under the continents. Below it begins the mantle of hot, viscous rock, reaching down to a depth of about 2900 km, which makes it the thickest shell. Deeper still comes the outer core of liquid iron and nickel, reaching down to about 5150 km. Right at the centre sits the inner core, a solid ball of metal extending to the midpoint at a depth of 6371 km.

The deeper, the hotter

On the way inward, the temperature keeps rising. Even in deep mines, workers notice it is much warmer down there than at the surface. In the inner core the temperature is about 6000 °C, roughly as hot as the surface of the Sun. The inner core stays solid anyway because of the enormous pressure: it squeezes the metal together so hard that it cannot melt.

The magnet in the deep

In the outer core, liquid iron slowly flows around. These currents act like a giant dynamo and generate the Earth's magnetic field. Without this field no compass needle would point north, and the charged particles from the Sun would hit our atmosphere unchecked and strip it away bit by bit. So the iron core deep below you protects you every day while you live your life on the thin crust.