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

Research shows there are little-known chapters in that history, deep within Earth's past. In fact, 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," Australian National University geophysicist Joanne Stephenson explained in 2021.

Our knowledge of what lies beneath Earth's crust has been inferred mostly from what volcanoes have divulged and what 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 1 percent of Earth's total volume.

But a few years ago, Stephenson and colleagues found evidence Earth's inner core may actually have two distinct layers.
"It's very exciting – and might mean we have to re-write the textbooks!" Stephenson explained at the time.