about 2,900 km (1,800 mi) below the surface, where there is an abrupt change in the speed of seismic waves (generated by earthquakes or explosions) that travel through Earth. At this depth, primary seismic waves (P waves) decrease in velocity while secondary seismic waves (S waves) disappear completely. S waves shear material, and cannot transmit through liquids, so it is thought that the unit above the discontinuity is solid, while the unit below is in a liquid or molten form. The discontinuity was discovered by Beno Gutenberg (1889-1960), a seismologist who made several important contributions to the study and understanding of the Earth's interior. The CMB has also been referred to as the Gutenberg discontinuity, the Oldham-Gutenberg discontinuity, or the Wiechert-Gutenberg discontinuity. In modern times, however, the term Gutenberg discontinuity or the "G" is most commonly used in reference to a decrease in seismic velocity with depth that is sometimes observed at about 100 km below the Earth's oceans. See also Core–mantle differentiation Ultra low velocity zone References External links Earth's Core–Mantle Boundary Has Core-Rigidity Zone Mineral phase change at the boundary Superplumes at the boundary About.com article on the name of D″ Geophysics Structure of the Earth