James Stewart

The reason he could see them was, being far from the epicentre, differing speeds of the waves meant they arrived one at a time.

The first arrivals were later known as primary waves, pressure waves, or P waves, which are longitudinal.

They oscillate in the same direction as they're traveling in.

These were then followed by secondary, shear, or S waves, which are transverse.

Their oscillations are perpendicular to their direction of travel.

Finally, surface waves ripple through the Earth's crust, and it is these that tend to cause the most destruction.

It had already been established by this point that longitudinal waves can pass through both solids and liquids, but transverse waves can only pass through solids, and you can probably see where this is going.

Since the seismic waves had travelled through the inside of Earth to reach the seismograms, the resulting seismographs could yield information about what they passed through, namely, the structure of Earth itself.

This realisation would allow centuries of speculation to finally be put to rest.

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.

That's where our friends at Planet Wild come in.

They are a community-based nature protection organisation fighting back.

Think of them like crowdfunding, but for nature.