The Occidental Sahara shows a very much younger desert surface relief than does the oriental portion. The Occidental Sahara is a world unto itself, but it has always been more or less closely associated with the life of the Atlas. The Algerian Desert which occupies the eastern portion of the region, extending southward from Algeria and Tunisia to the elbow of the Niger, is by far the best-known part of the whole Sahara, because it has been under military occupation. At the eastern extremity of the Algerian Sahara there is a small group of oases which includes Ghadames and Ghat, and which is of indefinite affiliation. In order to utilize the available water reserves in the great majority of the oases of southern Algeria, considerable subterranean labor has been necessary. In this respect they resemble the Egyptian oases of the Libyan Desert.