James Stewart

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.

And so, when he finally got the chance to speak, he proposed something else.

His theory was that the density difference must be due to there being a different, much denser material hidden deep in Earth's interior.

As to what that material could be, well, the inspiration for his theories came from the heavens, in the form of iron meteorites.

These little clumps of metal have a density higher than the average of Earth, and Wiehert reasoned that our planet's interior must therefore contain a large iron core, dense enough to account for the difference between lighter surface rocks and the higher planetary average.

Now, this wasn't an entirely new idea, but not only had we heard deployed mathematical reasoning rather than the purely theoretical work of his forebears, he'd also proposed it at just the right time.

A new technology was emerging that would finally allow the interior of the Earth to be studied.

Earth's core is so deep and the pressures and temperatures so intense that you can't simply drill down there and take a sample, although that would be nice.

Even today, our deepest man-made hole is the Kola Superdeep Borehole in Russia, and at more than 12km deep, that is still only a fraction of a percent, 0.2 to be exact, of the way there.

And so, in order to study the core, scientists rely on measurements taken here on the surface.