The Sahara Pump Theory is one which is used to explain the various phases by which African flora and African fauna have left that continent to penetrate the Middle East and possibly, thereafter, the rest of the world. African pluvial periods are associated with a "wet Sahara" phase, in which large lakes and many rivers are found. During these periods, the Sahara becomes a Savannah grassland and African flora and fauna become common. During the following inter-pluvial dry arid period, the Sahara reverts to desert conditions usually as a result of the retreat of the West African Monsoon southwards. Evaporation exceeds precipitation, level of water in lakes like Lake Chad falls, and rivers become dry wadis. Flora and fauna previously widespread retreats northwards to the Atlas Mountains, sout