Mauritania lies in the western region of the continent of Africa, and is generally flat, its 1,030,700 square kilometres forming vast, arid plains broken by occasional ridges and clifflike outcroppings. It borders the North Atlantic Ocean, between Senegal and Western Sahara, Mali and Algeria. It is considered part of both the Sahel and the Maghreb. Approximately three-quarters of Mauritania is desert or semidesert. As a result of extended, severe drought, the desert has been expanding since the mid-1960s.