The lower mantle is part of the mantle layer of the Earth.

The mantle is the thickest layer of the Earth, sitting beneath the crust and above the core. It's divided into two parts:

* Upper Mantle: This layer is closer to the crust and is more rigid.

* Lower Mantle: This layer is deeper, hotter, and more fluid.

So, the lower mantle is not a separate layer, but rather a part of the larger mantle layer.