Approximately the size of the continental United States, the Sahara Desert dominates the geography of northern Africa. The Sahara stretches from the Atlantic Ocean in the west to the Red Sea in the east, and from the Mediterranean Sea in the north to the Sahel, a tropical savanna, in the south. In total, the Sahara stretches 3,500,000 square miles, covering 10% of the continent of Africa. It is considered to be the largest "hot" desert in the world. Only one river is present in the Sahara, that being the famous Nile. Most of the remaining water is stored in underground aquifers and oases on the surface. The Sahara encompasses at least some portion of the countries of Tunisia, Niger, Sudan, Algeria, Mali, Egypt, Chad, Libya, Morocco, and Mauritania. The harsh climate of the Sahara is the main reason why few people live in it, and most of the inhabitants are nomads who move from region to region throughout the desert.