James Stewart

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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.

Something was stopping them.

Now, remember I said that S-waves don't transmit through liquids?

Well, it was a small leap, therefore, to go from that fact to the idea that Earth's core must therefore be liquid iron.

It wasn't completely wrong, but there was something they had all missed, and it took a Danish scientist to find it.

In 1929, Inge Lehmann was chief of the seismological department at Denmark's State Geodetic University.

Her responsibilities involved keeping the instruments correctly adjusted, interpreting the seismograms and publishing the bulletins of the seismic stations.

But her job did not extend to original research.

So when she decided to embark on her own research project, she had to do so alone, without the typical team of assistants to help her.

But it was precisely because she was forced to trawl through so much data by herself that she noticed something odd about the P waves she was studying.

You see, if the Earth had a liquid core, P waves should travel down from an earthquake, reach that core and be refracted due to the liquid's properties, like light entering a prism.

The expected result would be a shadow zone where no P waves would appear because they had all in fact been refracted away.