In July of last year, Tesla and SpaceX founder Elon Musk let slip that he was working on the Hyperloop -- an ultra-fast mode of transportation that will get you from downtown Los Angeles to downtown San Francisco is under 30 minutes. This is a distance of roughly 340 miles, and would require speeds of around 700 mph -- close to the speed of sound. Best yet, though, Musk says the Hyperloop will only cost around $6 billion -- compared to the $60 billion of the proposed high-speed rail link connecting the two cities. So far, so good, except for one niggling issue: We have absolutely no idea how Musk will construct the Hyperloop.