The thickest part of the lithosphere is found underneath the continents. This area is called the continental lithosphere.

Here's why:

* Continental crust is thicker than oceanic crust. It can be up to 70 km thick, compared to oceanic crust which is around 7 km thick.

* Lithosphere is made up of the crust and the uppermost part of the mantle, which is rigid and brittle.

* The continental lithosphere is thicker because it includes both the thicker continental crust and a portion of the upper mantle.

Therefore, the thickest part of the lithosphere is located beneath continents.