During the nineteenth century, the nature of the Earth’s interior was a matter of fierce and fascinating debate (p. 1). However, the nature of rocks deep below the surface was unknown, so a lack of evidence made it impossible to the gauge the worth of rival ideas. In 1906, Richard D. Oldham observed that compressional seismic waves (P waves) slow abruptly deep within the Earth and can penetrate no further. This was strong evidence in favour of a liquid core. Three years later, Andrija Mohorovi ∨cic´ (1909) noticed that the velocity of seismic waves leaps from about 7.2 to 8.0 km/s at around 60 km deep. He had discovered the ‘Moho’ seismic discontinuity that marks the crust-mantle boundary. In 1926, Beno Gutenberg obtained evidence for a seismic discontinuity at the core-mantle boundary. During the 1950s, world-wide records of blasts from underground nuclear detonations confirmed the presence of this, the Gutenberg discontinuity. Subsequent studies of the Earth’s seismic properties, using seismic waves propagated by earthquakes and by controlled explosions to ‘X-ray’ the planet (seismic tomography), has revealed a series of somewhat distinct layers or concentric shells in the solid Earth, each with different chemical and physical properties (Figure 2.1).