James Stewart

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.

Now thankfully there's one way we can get particularly insightful information, and that is from earthquakes.

By the 19th century, increased interest in seismic activity had prompted the creation of a device to measure them scientifically.

Yeah, this was the modern seismograph, and it looks something like this.

For the first time, the analysis of seismic events weren't based on eyewitness observation and hearsay, but on quantifiable data, which we love here on the channel.

But while the device itself was unquestionably a scientific breakthrough, the real magic happened when the invention spread across the globe.

As seismographic networks expanded, scientists found they could detect earthquakes happening as far away as the other side of the planet.

In 1897, the so-called Great Earthquake erupted at Shillong Plateau in India.

Shocks were felt in Calcutta and even as far away as Burma.

But seismographs detected this earthquake much, much further away, in Europe, where the seismograms they created were pored over by British geologist Richard Dixon Oldham.

Leader of the Geological Survey of India, Oldham noticed that the seismogram showed three distinct wave shapes.

In his papers, he compared the data found from this and several other earthquakes and found that they too showed these same three waves.