Andrija Mohorovicic found out that the velocity of seismic waves changes and increases at a distance of about 50km below the Earth's surface. This led to the idea that there is a difference in density between the Earth's outermost layer which is the crust and the layer that lies below it also known as mantle. The boundary between the two layers is called Mohorovicic discontinuity or Moho for short. :)
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