Those of you who haven’t managed to forget everything that was hammered into you in school may recall that our planet consists of three major layers. Much like an eggshell, Earth has a thin, rigid outer layer called the crust which houses all known life. This sits atop the thickest layer, the mantle, which consists of semi-solid rock and minerals. Below this is the heart of our planet: the hot, metallic core, which is split into two layers—the liquid outer and solid inner core. But interesting new research hints that the story may not end there, as scientists now have evidence to suggest that the inner core may have an inner core of its own.

The intriguing discovery was made by an international team of researchers from the University of Illinois and Nanjing University who have adapted earthquake-measuring technology to probe the deepest parts of our planet. Their results, which have been published in Nature Geoscience, indicate that there is a distinct region at the center of the inner core that differs from the rest in terms of the structure of the iron crystals present.

Scientists are interested in Earth’s core because revealing its characteristics could yield important insight into the history of Earth. “Even though the inner core is small—smaller than the moon—it has some really interesting features,” study author Xiaodong Song said in a news release. “It may tell us about how our planet formed, its history, and other dynamic processes of the Earth. It shapes our understanding of what’s going on deep inside the Earth.”

Because scientists can’t drill thousands of miles through Earth to reach the core, its precise composition remains a mystery. But there are indirect ways to peer down through our planet, which have helped us gain insight into what lies at Earth’s heart.

For the present study, researchers have been analyzing so called seismic coda, which are scattered waves that emanate from the source of an earthquake in a variety of directions. These waves vibrate through the different layers of Earth during an earthquake, much like how a bell rings after being struck by a hammer. By studying how these echoes change as they travel through and bounce off the different layers of Earth, scientists can glean information about their structure and composition.

This new data set revealed that our planet’s inner core, which was believed to be a solid ball primarily composed of nickel and iron, actually has a distinct inner core which seems to be around half the diameter of the entire inner core. If you stood on the North Pole, looking down, the iron crystals in the outer part of the inner core would be aligned vertically north to south. The crystals in the “inner inner core,” however, were flipped and point east-to-west, horizontally.

Alongside being aligned differently, researchers found that the iron crystals in the different layers also behaved differently, which could suggest that the inner and outer-inner cores may have formed under different conditions. Furthermore, the findings may indicate that the inner core experienced a dramatic change early in our planet’s history, which would tell an interesting story of how Earth has evolved.

[Via University of Illinois, Nature Geoscience, BBC News and Gizmag]