Today: electric cars. Tomorrow: the sound barrier? Elon Musk, the entrepreneur behind PayPal, Tesla Motors and SpaceX looks to be setting his sights next on express passenger air travel. Musk, frustrated that no options have risen to replace the defunct Concorde, suggested that once his current projects are more established he might be the one to do it. His "ultimate form of transport" would be a supersonic, electric-powered aircraft that could take off vertically like a space shuttle, with no need for land-gobbling runways. And that's not all: on Monday, he's set to reveal plans for a super-fast intercity transport system called Hyperloop. With air, space, land and loops under his belt, what's left to conquer?