It’s well known that the innermost part of Earth is made mostly of iron (about 85 percent). Nickel accounts for about 10 percent. That last 5 percent however, has remained a bit of a mystery.

A Japanese research team has been searching for that missing element for decades, and now believes that the final 5 percent is most likely made from silicon, reports the BBC.

They presented their results at the Fall Meeting of the American Geophysical Union in San Francisco.

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