We've drilled the Kola Superdeep Borehole (7,5 miles). We use seismology to observe deeper in the earth. In 1909, Andrija Mohorovičić, a Croatian seismologist, discovered the boundary between the crust and the mantle by observing the sudden increase of seismic waves as they passed from the crust to the mantle. Because of the sudden jump in wave velocity, he was able to infer that there must be a change in the composition in the rocks at that depth. The boundary between the crust and the mantle is generally known as the "Moho" since most scientists had trouble finding the special characters needed to write Mohorovičić's entire last name correctly. In 1912, Beno Gutenberg used observations of the sudden drop in P wave speed together with the observation of a "shadow zone" in which direct P and S waves do not arrive at seismograms a certain distance from the source to calculate that the depth of the core-mantle boundary must be at about 2900 km. In 1936, Inge Lehmann observed a second "shadow zone" within the core itself to discover the boundary between the inner core and the outer core. All 3 have documented their findings. @Derek Nelson
