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By using seismic waves to scan the inside of the Earth the way that doctors use ultrasound, a researchers at University of Illinois and Nanjing University in China have made a bizarre discovery. They’ve found that the Earth’s inner core has an inner core of its own, like a Russian nesting doll, that has different properties from what scientists have previously imagined.

Scientists once thought that the Earth’s core was composed of an outer core of liquid iron and a solid inner core of iron crystals that are aligned in a North-South direction. But, as it turns out, there’s also an inner-inner core, whose crystals point East-West. The inner-inner crystals behave differently also.

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“Even though the inner core is small – smaller than the moon – it has some really interesting features,” said University of Illinois geology professor Xiaodong  Song, co-author of a study just published in the journal Nature Geoscience. “It may tell us about how our planet formed, its history, and other dynamic processes of the Earth. It shapes our understanding of what’s going on deep inside the Earth.”

“The fact that we have two regions that are distinctly different may tell us something about how the inner core has been evolving,” Song said in a press release. “For example, over the history of the earth, the inner core might have had a very dramatic change in its deformation regime. It might hold the key to how the planet has evolved. We are right in the center – literally, the center of the Earth.”

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Cambridge University professor Simon Redfern told BBC News that the new study is significant, because it means that “something very substantial happened to flip the orientation” of the inner-inner core. He said other studies suggest that the Earth’s magnetic field underwent a major change in the distant past, switching alignment between the equator and the polar axis.

“It could be that the strange alignment Prof. Song sees in the innermost core explains the strange palaeomagnetic signatures from ancient rocks that may have been present near the equator half a billion years ago,” Redfern told BBC.

The Earth’s inner core has its own inner-inner core. Credit: Lachina Publishing Services, via University of Illinois