The Lehmann discontinuity is an abrupt increase of P -wave and S -wave velocities at the depth of 220 km (140 mi) in Earth's mantle, discovered by seismologist Inge Lehmann. The thickness is 220 km . It appears beneath continents, but not usually beneath oceans, and does not readily appear in globally averaged studies. Several explanations have been proposed: a lower limit to the pliable asthenosphere, a phase transition, and most plausibly, depth variation in the shear wave anisotropy.