Structure of the Earth
The moho, properly called the Mohorovičić discontinuity, is the boundary between the Earth's crust and the mantle. It was discovered by  Croatian seismologist Andrija Mohorovičić in 1909. He discovered that seismograms of earthquakes showed two kinds of seismic waves. There is a shallow slower wave which arrives first, and a deep faster wave which arrives second. He reasoned that the deeper wave changed speed as it got just below the mantle. The reason it went faster was that the material of the mantle was different from that of the crust.