The Sahara conceals large quantities of water stored at depth and inherited from ancient times. A recent study by the IRD and its partners has just shown that this groundwater is not entirely fossil, but resupplied every year. Using a method based on data obtained by satellite, scientists estimated the variations in the volume of water lying under the northern Sahara desert: the current rate of recharge is on average 1.4 km3 per year, for the period 2003-2010. This represents 40% of withdrawals, mainly for irrigation to support the oasis economy. The inputs therefore do not compensate for the withdrawals, but their existence means that these transboundary aquifers, the main water resource of semi-arid regions in Algeria and Tunisia, could be managed sustainably.