Elon Musk, the billionaire behind the Hyperloop, a concept for high-speed travel, has described a plan laid out by John Gardi as the "closest guess," according to NBC News.

Musk first mentioned the Hyperloop over a year ago, and now he is starting to share some information about the idea. It's a "fifth mode" of transportation, unlike the common modes that include cars, planes, trains and boats.

"It would work better than a high-speed rail, or a plane, between the right city pairs, like San Francisco and L.A., or New York-Boston," Musk told CNBC.

He estimates a trip between San Francisco and Los Angeles would take about half an hour. This would require a speed of approximately 700 mph. He also estimates it would cost a tenth of the cost of the $68 billion high-speed rail system currently proposed for California.

It's a "cross between a Concorde, a rail gun and an air hockey table," Musk said. Vehicles would travel "effectively faster than the speed of sound."

The Hyperloop "does involve a tube, but not a vacuum tube...not frictionless, but very low friction."

The design would work optimally with cities that are less than 1,000 miles away from each other.

"Once you get to the 1,000-mile range, you should just take a supersonic plane," Musk said.

"Pulled all nighter working on Hyperloop (as did others)," Musk, the head of SpaceX and Tesla, wrote on Twitter. "Hopefully not too many mistakes."

He also said that mentioning the Hyperloop at all was a mistake because he can't take on the challenge at the moment.

"I think I kinda shot myself in the foot by ever mentioning the Hyperloop, because I'm too strung out," Musk told Tesla investors last week. "Obviously, I have to focus on core Tesla business, and SpaceX business, and that's more than enough."

Musk hopes someone else will pick up where he left off on the idea, and do the work to execute. If that doesn't happen, he said he might pick it up again in a few years.