The Sahara, which is the largest desert of the world, covers more than 25% of the total area of Africa. It is thought that long ago, it was a fertile land, which gradually turned into a desert as the climate changed and the people failed to take proper care of their land. The soil is still fertile, but few plants grow there due to lack of fresh water. The Nile Valley or area near the Nile River is also a part of Sahara but instead of being a desert it is one of the most fertile regions in the world, as it is irrigated by the great Nile River. Small oases exist throughout the Sahara, but usually it is very difficult to obtain water. At some places, the wells have to be dug 1000 meters deep. Morocco, Algeria, Tunisia, Libya, Egypt, Sudan and some other countries share this vast desert.