If that's really the case, pumping it out sounds like a fool's errand, because it'll just run out. Either that or it's so vast that consumption by millions of people doesn't make a dent in the overall water level.
Cyclical changes in Earth's rotation pushes the North African Monsoon up to the Sahara for a few thousand years every 20k years or so. During this period, the area is much wetter, and over millions of years, very large amounts of water have been deposited in all the areas from where groundwater doesn't drain away.
The source of water for the GMMR is the Nubian Sandstone Aquifer System, which has an estimated 150,000 km³ of water in it, of which 2.4 km³ is pumped out every year.
