The layered Earth
"During the nineteenth century, the nature of the Earth's interior was a matter of fierce and fascinating debate. All theories were hampered by a lack of evidence -- the nature of rocks deep below the surface was unknown. In 1906, Richard D. Oldham observed that compressional seismic waves (P waves) slow abruptly deep with the Earth and can penetrate no further. This was strong evidence in favor of a liquid core. Three years later, Andrija Mohorovicic noticed that the velocity of seismic waves leaps from 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. This, the Gutenberg discontinuity, was confirmed during the 1950s when world-wide records of blasts from underground nuclear detonations were scrutinized. Subsequent studies of the Earth's seismic properties, using seismic waves propagated by earthquakes and by controlled explosions to 'x-ray' the planet (a technique called seismic tomography), have revealed a series of somewhat distinct layers or concentric shells in the solid Earth. Each shell has different chemical and physical properties..."