Scientists say they have gained new insight into what lies at the very centre of the Earth.
Research from China and the US suggests that the innermost core of our planet has another, distinct region at its centre.
The team believes that the structure of the iron crystals there is different from those found in the outer part of the inner core.
The findings are reported in the journal Nature Geoscience.
Without being able to drill into the heart of the Earth, its make-up is something of a mystery.
So instead, scientists use echoes generated by earthquakes to study the core, by analysing how they change as they travel through the different layers of our planet.
Prof Xiaodong Song, from the University of Illinois at Urbana-Champaign said: "The waves are bouncing back and forth from one side of the Earth to the other side of the Earth."
Prof Song and his colleagues in China say this data suggests that the Earth's inner core - a solid region that is about the size of the Moon - is made up of two parts.
The seismic wave data suggests that crystals in the "inner inner core" are aligned in an east-to-west direction - flipped on their side, if you are looking down at our planet from high above the North Pole.
Those in the "outer inner core" are lined up north to south, so vertical if peering down from the same lofty vantage point.
Prof Song said: "The fact we are discovering different structures at different regions of the inner core can tell us something about the very long history of the Earth."
The core, which lies more than 5,000km down, started to solidify about a billion years ago - and it continues to grow about 0.5mm each year.
The finding that it has crystals with a different alignment, suggests that they formed under different conditions and that our planet may have undergone a dramatic change during this period.
Commenting on the research, Prof Simon Redfern from the University of Cambridge said: "Probing deeper into the solid inner core is like tracing it back in time, to the beginnings of its formation.
"People have noticed differences in the way seismic waves travel through the outer parts of the inner core and its innermost reaches before, but never before have they suggested that the alignment of crystalline iron that makes up this region is completely askew compared to the outermost parts.
"If this is true, it would imply that something very substantial happened to flip the orientation of the core to turn the alignment of crystals in the inner core north-south as is seen today in its outer parts."
He added that other studies suggest that the Earth's magnetic field may have undergone a change about half a billion years ago, switching between the equatorial axes 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," he added.
"For the moment, however, the model proposed in this paper needs testing against other ways of analysing the seismic properties of Earth's innermost core, since no other researchers have previously considered evidence for the same conclusions in their studies."
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