Also Moho or M-discontinuity; Boundary that separates the Earth's crust and mantle, marked by a rapid increase in the speed of earthquake waves. It follows the variations in the thickness of the crust and is found approximately 32 km/20 mi below the continents and about 10 km/6 mi below the oceans. It is named for the Yugoslav geophysicist Andrija Mohorovicic (1857–1936), who suspected its presence after analyzing seismic waves from the Kulpa Valley earthquake 1909.