The crust is between 6km and 70km thick depending on where you measure it. The deepest mine in the world is only 3.3km deep, and nobody has ever been able to drill down further than 15km. Our knowledge of everything that exists below this depth is worked out by scientists using evidence from earthquakes, volcanoes and meteorites.
To put the thickness of the crust into context, if you had a football to represent the Earth, the crust would be the thickness of a postage stamp.
The surface of our planet is relatively cold, so the crust is rocky and brittle and can fracture during earthquakes. Although the crust was thought for many years to be a solid and fixed surface, it is not actually a continuous layer of rocks but consists of large masses, called plates, which are free to drift slowly across the surface of the planet.
There are two types of crust - Continental and Oceanic.
Continental crust is less dense ( lighter), much older and much thicker than oceanic crust. It floats on the mantle rather like an iceberg floats in water. There is a noticable difference in the thickness of the two types of crust. Continental crust is between 35 and 70km thick, and up to 3500 million years old. The two most important components of continental crust are Silicon and Aluminium, and it has an average density of 2.7g/cm3.
Oceanic crust is much thinner, generally about 6km thick, and nowhere is it older than 200 million years. The two most important components of oceanic crust are Silicon and Magnesium. It is more dense than continental crust, with an average density of 3g/cm3.
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