The Nile and Niger rivers, both fed by rains outside the desert, are the only permanent rivers in the region. Water is present at or just below the surface gravel in wadis (intermittent streams) that radiate from the mountain massifs, in scattered oases where the water table comes to the surface, and at greater depths in huge underground aquifers. The aquifers are believed to be filled with water dating from the Pleistocene epoch, when the Sahara was much wetter than it is today and had very large lakes. The more than 20 lakes (called chotts in the north) and areas of salt flats and boggy salt marshes are also considered relics from this pluvial period.
Important discoveries of minerals, oil, and gas have been made in the Sahara. There are huge oil and gas deposits in Algeria and Libya, but in most cases, inaccessibility has delayed exploitation. In searching for oil reserves, underground deposits of water also have been found. Extensive iron ore deposits are worked in the Fort Gouraud area of Mauritania. Salt is still mined, as in the past, at Taoudenni, Mali, and at Bilma, Niger, and is transported, as in the days of the great medieval kingdoms of W Africa, by camel caravans across the desert.