No one ever accused Elon Musk of not dreaming big. The co-founder of PayPal, Tesla Motors and founder of SpaceX is laying the groundwork for his next big attempt to transform mobility: "Hyperloop," a low-pressure tube that would zip passengers at speeds up to 800 miles per hour, more than twice as fast as the fastest bullet train.

On Tuesday, Musk tweeted out a proposed location for building a five-mile test track: "most likely Texas."

He also suggested a competition for students to test prototypes.

Musk unveiled plans for the Hyperloop in 2013, which would use pneumatic tubes to fire pods full of people from Los Angeles to San Francisco in less than 30 minutes. The massive vacuum tubes could be built above or below ground or water. Combined with magnetic levitation, the Hyperloop would provide a near friction-free means to travel long distances.

Musk has described the Hyperloop as "a cross between a Concorde, a railgun and an air hockey table," but it will be expensive. Hyperloop Transportation Technologies, a crowdsourced research group formed after Musk's proposal, estimated that the cost of a trans-California tube would be $7 billion to $16 billion.

Musk hasn't said much about the Hyperloop idea since he announced it in 2013, perhaps because he was tied up with multiple SpaceX launches and the development of the Tesla Model X SUV. Proof of the concept would be the first step, but the bigger hurdle would be similar to that challenging high-speed rail: securing the right-of-way for the straight lines required for such high speeds.