What is the Mantle?
A mantle is part of a rocky planet or another rocky body that is large enough. The mantle is located right under the crust of the earth and on top of the outer core. When you go outside you are stepping on the earth's crust, but under that crust is a vast ocean of rock. This vast ocean is the mantle, and the "ocean" is actually molten rocks. The mantle extends almost 2900 km thick, which is way more deep than the earth's crust! As you can see in this picture, the mantle is divided into 2 parts, the upper mantle and the lower mantle.
http://backreaction.blogspot.com/2010/06/diamonds-in-earth-science.html