James Stewart

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.

Data from a series of earthquakes that took place in close proximity at the South Sandwich Islands and also, crucially, a seismic station that had reliably monitored them for 32 years at College, Alaska.

By comparing the readings, they found that from 1967 to 1995, these similar waves were taking a faster and faster path through the Earth's core.

They had their answer.

Earth's core must have rotated at a different speed to the rest of the planet.

It was the first observational evidence of inner core rotation, and they calculated that it was spinning faster than its surroundings.

And now, with the technique proven, well, the floodgates opened.

In the last couple of decades, some studies have been published that agree the core rotates faster, while others have concluded that it spins more slowly.