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If you journeyed to the centre of the Earth, what would you see on the way down? After breaking through the crust there’s the mantle, a layer of mostly solid rock that constantly flows like a glacier. Below that, there’s the outer core made of liquid iron and nickel. Then you come to the inner core, a solid ball of metal. And now, researchers at The Australian National University (ANU) have found a hint of another core hiding inside that!

How do we know what’s below the surface? Even the deepest holes we’ve dug are still in the crust, and there are only a few places in the world where the mantle is exposed. To learn about deeper layers, scientists must study something that travels deep inside the Earth and then returns to the surface. Vibrations that make the entire planet ring – earthquakes!

Researchers use a global network of earthquake receivers to record these vibrations. They can use this information to work out where an earthquake happened, as well as what’s between the earthquake and the receiver. Every time there’s an earthquake, scientists learn more about what’s down below.

Recently, ANU researchers used an innovative mathematical technique to help understand this earthquake data. By using a search algorithm to create thousands of models, they found hints of a new layer, an innermost inner core.

These clues come from the time the earthquake wave takes to travel through the inner core. Earth’s inner core has a preferred direction of travel! Earthquake vibrations travelling parallel to the rotation axis move faster than those going along the equator.

The best model for the data has an innermost inner core with a slow direction that isn’t parallel to the equator. Instead, it’s at an angle of about 54 degrees!

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