Walter Munk headed a project to drill to the bottom of it in 1961, but this plan was discontinued five years later.  
Walter Munk headed a project to drill to the bottom of it in 1961, but this plan was discontinued five years later.  It results in refraction and the increased speed of seismic waves, both of which indicated its presence, and it lies as low as 75 kilometers underneath the continents and as high as five kilometers below the ocean floor.  Discovered in 1909, a year after the recording of an earthquake in the Kulpa Valley in Zagreb, FTP, name this large boundary between the Earth's crust and upper mantle, named for its Croatian discoverer.
Walter Munk headed a project to drill to the bottom of it in 1961, but this plan was discontinued five years later.  
Mohorovicic discontinuity (or Mohorovicic layer; prompt on "crust" in the first sentence)