After months of hints, tech entrepreneur Elon Musk unveiled his plan for the “hyperloop,” a transportation system he believes will be an alternative to airplanes and trains. Passengers would sit in a capsule that gets shot through an elevated tube. Contrary to initial speculation, the air in the tube is low pressure but not completely vacuumed out, and the pods are accelerated magnetically but also have an air jet similar to the Concorde. High tailwinds in the tube mean that the pods don’t cross the sound barrier relative to the air, avoiding a sonic boom. Musk says the hyperloop could take you from L.A. to San Francisco in about 30 minutes and would cost $6 billion to build, far cheaper than California's $70 billion high-speed-rail plan. Unfortunately, Musk is only posting the plan for feedback and says he's too busy with his spaceship and electric car businesses to build it himself.