Cape Town is withering. If current projections hold, the South African city of 4 million will run out of water on May 11, known as Day Zero. It’s been three long years of drought—we're talking a once every 1,000 years kind of problem that Cape Town's water infrastructure just wasn't built for. The irony is that a whole sea of water laps at the shores of the coastal city. But if you wanted to drink it, you’d have to build an expensive, energy-intensive desalination facility.