James Stewart

Put simply, Oldham noticed that waves that had travelled a long way to distant seismographs seemed to travel much slower than the six kilometres per second it usually moved through the mantle.

To explain this, he concluded that they had traversed a central core composed of matter which transmits them at a slower speed, three kilometres per second to be exact.

He calculated the size of this core to be four tenths of the Earth's radius, deduced that it bends earthquake waves and that it behaves fundamentally differently to the rest of the Earth's interior.

But he did not speculate as to what this core might be made of, refusing to go beyond what he could prove with data.

And so by the early 20th century, you had a theory that the Earth had an iron core and apparent proof that some sort of core did in fact exist.

We were finally beginning to crack the inner structure of our planet.

And yet, despite these great strides, these pioneers missed something.

Sometimes I get so focused on all the awesome stuff happening below our feet, I forget that the stuff on land also needs a bit of love too, perhaps now more than ever.

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As seismographs continued to spread around the world and more and more data was gathered for scientists to pour over, they noticed that S-waves didn't register on some seismographs.