Moho
Moho, or Mohorovicic Discontinuity, a zone in the earth below which shock waves travel more rapidly than they do above it. The Moho is thought to be the boundary between the earth's crust and the underlying mantle.
The Moho was discovered in 1909 by Andrija Mohorovii, a Croatian geophysi-cist, during studies of an earthquake. Since then many studies of the Moho have been made. The Moho is studied indirectly by tracing primary seismic waves (shock waves from earthquakes). Studies using shock waves from small explosions are also made. The speed at which the waves travel increases from about 4 miles (6.4 km) per second to about 5 miles (8 km) per second at the Moho. The Moho is 20 to 25 miles (32 to 40 km) below most landmasses, and, on the average, 6 miles (10 km) below the ocean floor; in some places, it is only 2 12 miles (4 km) below the ocean floor.
In 1957 United States scientists began Project Mohole, a program to drill through the ocean floor to obtain rock samples from the Moho and the underlying mantle. The project, financed by the National Science Foundation, was hampered by administrative problems and opposition to its increasing cost. It was halted in 1966.