By JUSTIN PRITCHARD

Associated Press

LOS ANGELES (AP) -- Imagine strapping into a car-sized capsule and hurtling through a tube at more than 700 mph -- not for the thrill of it, but to get where you need to go.

On Monday, billionaire entrepreneur Elon Musk unveiled a transportation concept that he said could whisk passengers the nearly 400 miles between Los Angeles and San Francisco in 30 minutes -- half the time it takes an airplane.

If it's ever built.

His ``Hyperloop'' system for travel between major cities is akin to the pneumatic tubes that transport capsules stuffed with paperwork in older buildings.

In this case, the cargo would be people, reclining for the ride.

The system would feature a large, nearly air-free tube. Inside, capsules would be pulled down the line by magnetic attraction.

Capsules would float on a cushion of air they create - like an air hockey table in which the puck produces the air instead of the surface. To minimize friction from what air is in the tube, a powerful fan at the front of each capsule would suck air from the front to the rear.

"Short of figuring out real teleportation, which would of course be awesome (someone please do this), the only option for super fast travel is to build a tube over or under the ground that contains a special environment," Musk wrote in his proposal, posted online.

On a conference call Monday, Musk said that if all goes right, it could take seven to 10 years for the first passengers to make the journey between California's two biggest metro areas. He put the price tag at around $6 billion - pointedly mentioning that would be about one-tenth the projected cost of a high-speed rail system that California has been planning to build.

Like that bullet train, the Hyperloop didn't take long to attract skepticism.

Citing barriers such as mountains and cost, one transportation expert said that while Musk's idea is novel, it's not a breakthrough.

"I don't think it will provide the alternative that he's looking for," said James E. Moore II, director of the transportation engineering program at the University of Southern California.

Monday's unveiling lived up to the hype part of its name.

Musk has been dropping hints about his system for more than a year during public events, mentioning that it could never crash and would be immune to weather.

Coming from almost anyone else, the hyperbole would be hard to take seriously. But Musk has a track record of success. He co-founded online payment service PayPal, electric luxury carmaker Tesla Motors Inc. and the rocket-building company SpaceX.

Musk has said he is too focused on other projects to consider actually building the Hyperloop, and instead would publish an open-source design that anyone can use or modify.

That's still the case, he said Monday, but added that if no one else steps forward he might build a working prototype. That would take three or four years, he said.

AP National Writer Martha Mendoza contributed to this report from San Jose, Calif.