James Stewart

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.

Yet after an earthquake in New Zealand, she discovered P waves in the shadow zone, precisely where they shouldn't be.

In a paper titled simply P, she attempted to explain what she had seen and reconcile it with existing observations, concluding that the core had two parts.

An outer core that was liquid and so stopped the S waves, but crucially, also an inner core that was solid, so could transmit some P waves to the shadow zone where she'd seen them.

As a woman working in the male-dominated world of science, Inga had to fight to have her conclusions heard.

But she would turn out to be absolutely correct.

That solid inner core was the final piece of the puzzle.

However, as is often the way with scientific research, this discovery simply paved the way for more questions.

Since the solid inner core is held within a liquid outer core, does it necessarily spin in sync with the rest of the planet?

Or could it be rotating at its own speed?