<human>:  What are the major geographical features of the Sahara Desert?
<bot>: The Sahara Desert is the largest desert in the world and it is located in the Northern part of Africa. Its major geographical features include: vast sand dunes, gravel plains, and rocky plateaus. Other features include oases, wadis, and limestone plateaus known as hammadas. The highest point in the Sahara Desert is the volcano, Mount Koussi, with an elevation of 3,415 meters.