For eons, the earth has had the same amount of water—no more, no less. What the ancient Romans used for crops and Nefertiti drank? It’s the same stuff we bathe with. Yet with more than seven billion people on the planet, experts now worry we’re running out of usable water. The symptoms are here: multiyear droughts, large-scale crop failures, a major city—Cape Town—on the verge of going dry, increasing outbreaks of violence, fears of full-scale water wars. The big question: How do we keep the H20 flowing?