|There's probably a fifth layer. (forplayday/iStock/Getty Images)|
It may be time to rewrite the textbooks, as scientists have discovered the Earth has a 'fifth layer' in the form of an innermost inner core at the centre of the planet.
It's an idea worthy of a Jules Verne novel; a mysterious layer at the centre of our planet.
Now researchers from The Australian National University (ANU) have confirmed the existence of the Earth's "innermost inner core".
Lead author of the study, PhD researcher Joanne Stephenson, says while this new layer is difficult to observe, its distinct properties may point to an unknown, dramatic event in the Earth's history.
"We found evidence that may indicate a change in the structure of iron, which suggests perhaps two separate cooling events in Earth's history," Ms Stephenson said.
"The details of this big event are still a bit of a mystery, but we've added another piece of the puzzle when it comes to our knowledge of the Earths' inner core."
Ms Stephenson says that investigating the structure of the inner core can help us understand more about the Earth's history and evolution.
"Traditionally we've been taught the Earth has four main layers: the crust, the mantle, the outer core and the inner core.
"The idea of another distinct layer was proposed a couple of decades ago, but the data has been very unclear.
"We got around this by using a very clever search algorithm to trawl through thousands of the models of the inner core.
"It's very exciting - and might mean we have to re-write the textbooks!"
For the very least, Verne's famous novel may need a couple of extra pages.
'The details of this big event are still a bit of a mystery, but we've added another piece of the puzzle when it comes to our knowledge of the Earths' inner core,' explained Stephenson.
Until recently our understanding of the deepest depths of our world have been through a combination of volcanic eruptions and seismic waves.
They are indirect observations but allowed geologists to determine that the inner core reaches temperatures of more than 5,000 degrees Celsius.
The inner core is also relatively small, making up just 1% of the Earth's volume and existing as a single body, with the outer core surrounding it, mantle surrounding that and the crust surrounding the mantle.
The structure of Earthʼs deep inner core has important implications for core evolution, since it is thought to be related to the early stages of core formation.
The research has been published in the Journal of Geophysical Research: Solid Earth.