The Earth's core is solid. How do we know?

You might think the Earth is molten inside, but deep within lies a solid core 1200 kilometers in diameter.

We know this thanks to earthquakes. When an earthquake occurs, two types of waves are generated: P-waves, which compress everything and travel through both solid and liquid parts, and S-waves, which move up and down perpendicular to their direction, like a wave on a rope. S-waves only travel through solids.

Danish seismologist Inge Lehmann, who experienced an earthquake only once in her life at age 15, discovered in 1936 that certain waves from earthquakes on the opposite side of the world were missing. Her model revealed that these waves reflected differently off a solid inner core.

By listening to Earth's seismic waves, we uncover the secrets of its heart.