The Hyperloop was first posed in July of 2012 by SpaceX and Tesla Motors founder and futurist Elon Musk, who unveiled the details of his idea in a blog post just over a year later. Here's how it's supposed to work: people (or cars, even) sit inside of aluminum pods, which themselves are enclosed in twin steel tubes that run along an elevated route. The pods would float on a cushion of air and accelerate on a series of magnets the way modern maglev trains do already. Regularly placed air pumps lower the pressure inside the tubes, which would reduce friction enough to make subsonic speeds possible—we're talking up to 800 miles per hour. Solar panels on top of the tunnels could generate the electricity the system needs.
Of course, plenty of thinkers have proposed big ideas about transportation in the past, but not many of them have come to fruition. The MIT Technology Review points out why the Hyperloop is different: "Although the design was ambitious to the point of being outlandish, none of its components were fundamentally unproven, something often overlooked."