If it were possible to drill a hole to the center of the Earth, about 1,800 mi (2,900 km) below the surface the drill bit would reach the Earth's core. First the drill would bore through the solid, relatively low density rock of the crust. Then anywhere from 4.5 to 30 mi (7 to 50 km) below the surface the bit would encounter the much denser rock of Earth's mantle. Finally it would reach the core, which consists of an outer molten layer beginning at about 1,800 mi (2,900 km) and then an inner solid crystalline mass starting at about 3,150 mi (5,100 km).

Sometimes called the centrosphere, or the Earth's innermost layer, the core was almost a complete mystery until the development of the science of seismology and seismic instruments. Seismographs reveal the nature of the vibrations, or seismic waves, produced during an earthquake. As seismologists learned more about seismic waves, they realized they could...