While most of us take the ground beneath our feet for granted, written within its complex layers, like pages of a book, is Earth’s history. Our history.

Now researchers have found more evidence for a whole new chapter deep within Earth’s past – Earth’s inner core appears to have another even more inner core within it.

“Traditionally we’ve been taught the Earth has four main layers: the crust, the mantle, the outer core and the inner core,” explained Australian National University geophysicist Joanne Stephenson.

Our knowledge of what lies beneath Earth’s crust has been inferred mostly from what volcanoes have divulged and seismic waves have whispered. From these indirect observations scientists have calculated that the scorchingly hot inner core, with temperatures surpassing 5,000 degrees Celsius (9,000 Fahrenheit), makes up only one percent of Earth’s total volume.

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Now Stephenson and colleagues have found more evidence Earth’s inner core may have two distinct layers.

“It’s very exciting – and might mean we have to re-write the textbooks!” she added.

The team used a search algorithm to trawl through and match thousands of models of the inner core with observed data across many decades about how long seismic waves take to travel through Earth, gathered by the International Seismological Center.

Read more: Science Alert