James Stewart

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?

To find out, scientists would require incredibly sophisticated seismographs, and they would soon have access to such instruments.

Although it would not be thanks to pure science alone, but to war.

Well, kind of.

By the second half of the 20th century, the world was in the midst of the Cold War.

But while the general public lived with the spectre of nuclear Armageddon, for seismologists it was proving to be a time of incredible excitement, as the United States instigated Project Vela Uniform.

Its aim was to develop a suitable system for detecting underground nuclear testing in the Soviet Union.

And what resulted was an investment of over half a billion dollars in today's money in seismographic technology and networks.

Crucial to the project's success was the ability to distinguish between the seismic waves produced by nuclear tests and those made by naturally occurring earthquakes.

And so by the time the Cold War ended, researchers at Harvard University had made breakthroughs using these seismographs.

They showed that seismic waves traveling along Earth's north-south axis through the inner core moved faster than those undertaking the journey in an east-west direction.

To explain this speed difference, they theorized that the Earth's core is anisotropic, meaning it has a crystalline structure aligned with Earth's rotation roughly along its magnetic field.