The earth's crust is between three to five miles deep under the oceans (oceanic crust) and about 25 miles deep under the continents (continental crust). This is very thin in comparison to the other layers of the planet.
The earth's crust is composed of vast pieces of rock called plates, which effectively float on the hotter, denser layer called the mantle. Temperatures at the deepest part of the crust can reach 1,600 degrees Fahrenheit, and it is at this depth that a layer called the Mohorovicic discontinuity, sometimes called the Moho, is found. This layer separates the crust from the hotter, softer mantle.