Chinese and the US researchers suggest that the innermost core of the Earth has another, distinct region at its center.
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.
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.”
He 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. Xiaodong 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.