James Stewart

This layer also appears solid, but is actually flowing very, very slowly, at the rate of a few centimetres a year.

This tiny but inexorable movement contributes to the plate tectonics that impact our surface world.

Dig deeper still and you reach the outer core, a sea of iron and nickel with its own heat-driven currents.

Its motion generates the Earth's magnetic field, which protects life on our planet from deadly solar particles.

Finally, suspended in this outer core is that primarily iron inner core.

Here, the temperature reaches 6,000 degrees Celsius.

Due to the immense pressures, the metal isn't liquid, but solid, forming a spheroid shape, like a slightly squashed sphere.

This is our planet's boiler room.

Its heat triggers the processes in the layers above.

driving those all-important convection currents in the outer core and ultimately contributing to the plate tectonics that shift the Earth's crust.

It's easy to take for granted that we understand our planet's structure, but in reality that knowledge was incredibly hard won.

It's one thing studying, cataloguing and theorising about the world we can see all around us, but quite another to investigate our planet's hidden interior.

So how do we know what's down there?

In Frankfurt, September 1896, darkness was falling as a 34-year-old Emil Wiechert waited for his chance to speak at a meeting of German scientists and physicists.

Clutched in his hands was his latest research, which proposed a solution to a problem that had plagued the scientific community for more than a century.

You see, the average density of planet Earth had been calculated at around 5.5 grams per centimetre cubed.

And yet, surface rocks have a density of around half that.

So where was that missing mass?

One theory was that the Earth gets progressively denser nearer the centre.

But that wasn't good enough for Wichert, who reasoned that molecules in a solid are already pretty densely packed, and even the compressional effects of high pressure wouldn't be enough to achieve the density required.