The Mohorovicic discontinuity (Moho) is the boundary between the Earth's lithosphere and mantle. It is identified by a change in the velocity of seismic waves passing through the Earth, and is named after the Croatian geophysicist Andrija Mohorovicic (1857–1936), who first recognized it in 1909.
The velocity change is explained by a change to more dense rocks in the mantle; above the Moho the waves travel at about 6.4 km/s (4 mi/s), whereas below they travel at 8.2 km/s (5.1 mi/sec). The depth of the Moho varies from about 5 km (3 mi) to 60 km (37 mi) below the Earth's surface; it is usually deeper beneath landmasses than beneath the sea. Initially interpreted as a sharp boundary, it is now known to be a wider zone in some locations and its exact nature remains a matter of scientific debate.
The Gutenberg discontinuity (discovered in 1912, by Beno Gutenberg) separates the mantle and the core, and is about 2,900 km (1,800 mi) deep. Earthquake and other shock waves are deflected at these boundaries.
The US project Mohole, designed to drill through the 'Moho,' was abandoned in 1966.