James Stewart

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.

Think of it like the grain on a plank of wood, and just as a plank of wood is easier to mill in one direction than the other, it's easier for earthquake waves to pass through the core on the north-south axis than the east-west.

Well, almost.

It turns out that this crystalline structure doesn't align precisely with the rotation of the Earth, but rather is tilted a few degrees off axis.

This was crucial because it gave scientists a potential way of determining how fast the inner core spins.

If it spins at a different rate to the rest of the planet, then that would continually change the way those crystals are aligned with the rest of the Earth, which would in turn affect the wave travel.

This realization gave birth to a method of assessing inner core spin that is still used to this day.

Now, working out whether Earth's core was spinning at its own rate wasn't easy.

Scientists recognised they needed data from multiple earthquakes that had happened in nearly the same spot, but far enough apart in time that the core could have moved.

Fortunately, they had decades of earthquake data thanks to the Cold War, and so the hunt was on for what they termed earthquake doublets.

Now, such doublets are incredibly rare.

Imagine hunting for identical twins when they're sat in different parts of a crowd at Wembley Stadium.

Fortunately, it was no longer the era of pen and paper, and with the increased digitisation of seismic data, what would have once been thousands of human hours was now the work of algorithms.

By 1996, a team of researchers at Columbia University had found what they were looking for.