X-Raying the Planet

Seismic waves have revealed Earth's entire internal structure, effectively giving us a medical scan of the planet:

• Crust — 5-70 km thick, the thin outer shell we live on, separated from the mantle by the Mohorovicic discontinuity
• Mantle — extends to 2,900 km depth, made of semi-solid silicate rock that flows very slowly over geological time
• Outer core — liquid iron and nickel from 2,900 to 5,150 km, confirmed by S-wave shadow zones
• Inner core — solid iron sphere about 1,220 km in radius, kept solid by extreme pressure despite temperatures exceeding 5,000 degrees C.