If this is true ...

“Deep beneath Africa, under some of the hottest and driest places on earth, lie vast reservoirs of fresh water. According to geologists, these ‘aquifers are some 410,000 cubic miles thick and contain 100 times the freshwater that exists on the continent’s surface.’ Tapping that water could transform dry and barren land in places like the Sahel into wet and fertile oases.” (Emphasis added.)

http://blogs.the-american-interest.com/wrm/2012/12/18/farming-the-sahara-and-the-end-of-malthusianism/