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Without understanding what it is, we often hear the phrase “down to the Moho”, meaning very very deep. The Mohorovicic Discontinuity, commonly called the “Moho” is recognized as the boundary zone between Earth's crust and the mantle.
This boundary marks a change in seismic-wave velocity from the crust to the uppermost mantle within the (lithospheric) plate. This boundary was discovered by Andrija Mohorovičić, a Croatian meteorologist-turned seismologist, after an earthquake in Croatia in 1909.

Graphics and animation by Jenda Johnson, Earth Sciences Animated
Co-authors: Jenda Johnson and Vince Cronin, Baylor University
Narrated by Mladen Dordevic, IRIS
Reviewed by Robert Butler, University of Portland, and Walter Mooney, US Geological Survey