The Mohorovičić discontinuity, also known as the moho, is the space between the earth's crust and mantle. The moho seperates oceanic crust from continental crust. No one has ever been or drilled deep enough into the earth to penetrate the moho. Drilling a well to that depth would be very expensive and dangerous due to the pressure and temperature. The moho is 5-10 km below the ocean floor and 20-90km below continents. It is estimated to be 0.2-3km thick.
The moho displays a resistant surface because it is made up of rocky material. This marks the place where seismic waves traveling down through the earth's crust speed up.
The Mohorovicic discontinuity was identified in 1909 by Andrija Mohorovicic. He is considered to be one of the founders of modern seismology.