The kingdom of Saudi Arabia has no permanent rivers or lakes. What it does have are wadis, valleys that are transformed into ephemeral rivers after storms, and aquifers. This image shows the spread of agriculture around the town of Wadi ad-Dawasir. The aquifer and irrigation have been used to green up portions of the desert. The water table in this area has dropped as much as 6 meters (20 feet) per year, fast enough that scientists think the aquifer could be depleted within a few decades. It is the most stressed of the world’s thirty-seven largest aquifers.