We have never drilled beyond a few kilometres, yet we describe the Earth’s interior in detail. How do we know the structure of the Earth’s interior?

The deepest borehole ever drilled, the Kola Superdeep in Russia, reached barely 12 km, less than 0.2% of the Earth's radius. Almost everything known about the interior is inferred indirectly, mainly from the way earthquake waves travel through it.

Seismic evidence, the primary source

• Two wave types behave differently: P-waves pass through solids and liquids; S-waves pass only through solids.
• The S-wave shadow zone: S-waves vanish beyond about 103 degrees from an epicentre, proving the outer core is liquid.
• The P-wave shadow zone: P-waves are absent between roughly 103 and 143 degrees because they refract sharply on entering the core, fixing the core-mantle boundary at about 2,900 km.
• Discontinuities map the layers: The Mohorovicic discontinuity separates crust from mantle, the Gutenberg discontinuity mantle from core, and the Lehmann discontinuity the liquid outer core from the solid inner core.
• Velocity gradients reveal state: The low-velocity asthenosphere indicates partial melting.

Corroborating evidence

• Density and moment of inertia: Bulk density of about 5.5 g/cc against surface rocks of 2.7 g/cc requires a very dense core.
• Meteorites: Iron meteorites are thought to be fragments of the cores of shattered planetesimals, supporting an iron-nickel core.
• Geomagnetism: A self-sustaining magnetic field requires a convecting, electrically conducting liquid layer.
• Heat flow and volcanic material: Geothermal gradients and deep-sourced xenoliths and kimberlites sample mantle composition directly.
• Laboratory simulation: Diamond-anvil experiments reproduce core pressures and test candidate minerals.

The picture of the interior is therefore a convergent inference, seismology supplying the structure and gravity, magnetism, meteoritics and high-pressure physics supplying the composition.