Instead of continually revolving faster than Earth's spin, the solid inner core oscillates, changing direction every six years. This has consequences for our understanding of Earth's inner workings and can explain an oscillating fluctuation in Earth's day with a 5.8-year duration.

Geophysicist John E. Vidale of USC, Los Angeles, remarked, 'From our data, we can detect the Earth's surface movements relative to its inner core' (UCLA).

'Recent observations reveal that the inner core-spun slower from 1969-71 and then faster from 1971-74. As planned, day length rose and shrank. Two observations suggest oscillation.'

Although we've learned a lot about Earth's core in recent decades, we still don't know much. Everything we know comes from indirect observations, such as seismic waves bouncing across the planet.

It's still effective. Earth's inner core is probably a hot, compact ball of solid iron around 2,440 kilometers (1,516 miles) across, bigger than Pluto. It rotates faster than Earth, according to evidence.

Superrotation is 1 degree every year, according to 1996 research. Using data from 1970s underground nuclear experiments at Novaya Zemlya, Vidale and Wei Wang of UCLA recalculated the pace to 0.29 degrees per year.

In the current research, they added 1971 and 1969 Amchitka Island testing. It was bizarre. The findings revealed that Earth's inner core was sub-rotating rather than superrotating, by 0.1 degrees each year.

This matched oscillation, researchers said. When superrotating, the inner core slows before speeding up again. 'We expected the same rotation direction and rate as in past atomic testing, but saw the reverse. We were startled it was moving backward.'

The oscillation's six-year period fits other unknown oscillations. Earth's days vary by 0.2 seconds every six years, and so does its magnetic field. They match the amplitude and phase of Vidale and Wang's inner core hypothesis.

This implies unraveling more data could be difficult. The US Air Force's Large Aperture Seismic Array ceased in 1978, and underground nuclear testing is far less frequent.

Advances in sensor technology could imply that the detailed data needed to investigate Earth's deep core isn't far off; the results so far hint that Earth's insides are more complex than we thought.

'The inner core isn't fixed; it moves a couple of kilometers every six years,' Vidale remarked. 'Long-term, does the inner core move progressively, or is it mostly locked? This is a key step in understanding how the inner core formed and moves over time.'