According to decades worth of seismic activity, the inner core might have recently paused, and even be slowly reversing.

"Here, we show surprising observations that indicate the inner core has nearly ceased its rotation in the recent decade and may be experiencing a turning-back in a multidecadal oscillation, with another turning point in the early 1970s," the team write in their new paper.

Earth's inner core is a bit of a wild card. Because the solid inner core is sitting in a larger outer liquid core, it isn't completely beholden to what the outer planet (like the mantle) is up to.

Understanding how both the inner and outer core works is important, as both seem to relate to the generation of Earth's magnetic field. The spin of the inner core is driven by the magnetic field generated in the outer core and balanced by the gravitational effects of the mantle.

Knowing how the inner core rotates could illuminate how these layers interact. However, scientists have not yet been able to agree on the speed of this rotation, and whether it varies.

Scientists first thought that the inner core might be faster than the planet's surface. Then it was slower, while a study we covered last year suggested it could even be oscillating over a six year cycle.

Now, the new research is suggesting that it could be oscillating not in six year long cycles, but seven decade long cycles.

Yi Yang and Xiaodong Song analysed the difference in the waveform and travel time of seismic waves from near-identical earthquakes that have passed through the Earth's inner core along similar paths since the 1960s.

They found that since around 2009, paths that previously showed significant temporal variation have exhibited little change, which they suggest shows that the inner core rotation has paused.

The team looked further back in time to the late 60s and early 70s and suggested that this may be associated with a reversal of the inner core rotation as part of a seven-decade oscillation, as the previous turning point occurring in the early 1970s.

IN OTHER NEWS:

If this was the case, it would line up with some geophysical observations too - particularly the way the magnetic field changes over time, and variations to the length of an Earth day.

"These observations provide evidence for dynamic interactions between the Earth's layers, from the deepest interior to the surface, potentially due to gravitational coupling and the exchange of angular momentum from the core and mantle to the surface," the researchers write in their paper.

With such a shaky topic, this is unlikely to be the last we hear on the matter. The research has been published in Nature Geoscience.