The mantle is the largest of the geological layers that make up the Earth. It is found beneath the crust and is about 1,800 miles thick. Surrounding the core, the mantle makes up about 84 percent of the volume of the Earth.
The mantle is made up of mafic and ultramafic rock. These are rocks made largely of iron and magnesium, and mafic is a portmanteau of the words "magnesium" and "ferric." The rock in the mantle is very hot and can be viscous or molten depending on the temperature and the pressure.
The mantle is divided into two zones. The upper mantle contains the lithosphere and the asthenosphere beneath it. The rocks in the lithosphere are mostly rigid, while the rocks in the asthenosphere are more plastic.
There is a transition zone between the upper and lower mantle and a boundary between the lower mantle and the core. The thickness of this boundary can vary.
There is also a boundary between the crust and the mantle called the Mohorovičić discontinuity or the Moho. Despite decades of effort, geologists still have not drilled all the way to the Moho. It is about 3 miles beneath the crust at its shallowest point.