The Lehmann discontinuity is an abrupt increase of P-wave and S-wave velocities at the depth of 220 km (140 mi), discovered by seismologist Inge Lehmann.[2][3][4] The thickness is 220 km[citation needed]. It appears beneath continents, but not usually beneath oceans,[5] and does not readily appear in globally averaged studies. Several explanations have been proposed: a lower limit to the pliable asthenosphere, a phase transition,[6] and most plausibly, depth variation in the shear wave anisotropy.[7] Further discussion of the Lehmann discontinuity can be found in the book Deformation of Earth Materials by Shun-ichirō Karato.[8]