What is the Moho, the boundary between Earth's crust and what layer?

The Moho is the boundary between Earth's crust and the mantle. Its full name is the Mohorovičić discontinuity, named after Croatian seismologist Andrija Mohorovičić, who identified it in 1909.

Mohorovičić studied earthquake waves and noticed that some seismic waves arrived sooner than expected at distant stations. He concluded that the waves had crossed into a deeper material where they traveled faster. That change in seismic behavior marks the transition from crustal rock to the denser, hotter mantle.

The Moho is not at one fixed depth. Beneath oceanic crust it is commonly about 5–10 kilometers below the seafloor, while beneath continental crust it is often around 30–40 kilometers deep and can be deeper under mountain ranges. It is a seismic boundary rather than a visible underground surface. The mantle is solid on ordinary timescales, although it can flow very slowly over geological time.