Thousands of miles beneath our feet, Earth’s inner core – a solid sphere made of iron and nickel metals — spins at a different rate than the planet as a whole. Scientists have long known that this difference is a result of the inner core spinning faster, but recently researchers discovered that it might also pause and reverse direction. It turns out this is a regular cycle the inner core goes through on a 70-year rotational loop. As VICE reports, researchers from Peking University were astonished when they studied seismic waves from earthquakes that have taken similar paths through the Earth’s inner core since the 1960s. This allowed them to spot the tiny variations in the speed and direction of the inner core’s rotation over time, which is most obvious when comparing “doublet events” (repeating earthquakes with nearly identical waveforms at common receivers). The researchers found that this pattern reached a minimum around 2009, suggesting that the inner core stopped rotating and might be in the process of switching direction again.

The research is published this week in the journal Nature Geoscience. Yi Yang and Xiaodong Song, the two Peking scientists behind the work, think the inner core goes through this seven-decade swing movement because the magnetic forces of a growing storm in the molten outer layer cause it to slow down in the northern hemisphere and speed up in the southern hemisphere. This asymmetry creates the Coriolis forces that drive the massive swirling lines and central eye of hurricanes.