Earth's entire metallic core is liquid.

Seismologists cannot see Earth’s core directly, but earthquake waves let them probe it.

One important clue comes from shear waves, or S-waves. These waves can travel through solid rock but not through liquid. Their disappearance on the far side of Earth showed that a large region deep inside the planet is liquid, and for a time it was reasonable to picture the metallic core as liquid throughout.

Some earthquake waves did not fit that simple picture. In 1936, Danish seismologist Inge Lehmann studied faint signals reaching places where a completely liquid core should have produced a deeper shadow. She proposed that the waves were being reflected and bent by a smaller body at the very center of Earth.

Later observations confirmed the basic structure: a liquid outer core surrounds a solid inner core.

The inner core is not solid because it is cool. Its temperature is comparable to the surface of the Sun. It remains solid because the enormous pressure at Earth’s center raises the melting point of its iron-rich material.

The old model was therefore only partly wrong. Much of the core really is liquid; the correction was the discovery of a solid center inside it.