Africa┐s Sahara Desert is the world┐s largest dry desert region. Covering an area of 5,500,000 square kilometres, the Sahara lies in Northern Africa stretching from the Atlantic ocean to the Nile river. It covers one quarter of Africa┐s land surface measuring 5,000 kilometres from east to west and 1,500 kilometres from the Mediterranean coast in the north to the Sahel in the south. The Sahara expands and contracts from one year to the next depending on rainfall. Africa┐s other great desert, the Kalahari Desert, is technically a semi arid zone and not an actual desert. It is a vast area of wind blown sands which form the longest continuous stretch of sand in the world. The Kalahari covers a 712,250 square kilometre region in south west Botswana and northern South Africa.