This week, Tesla Motors head Elon Musk unveiled his designs for the Hyperloop — a high-speed train that would whisk passengers between San Francisco and Los Angeles in 30 minutes at the incredible speed of almost 800 miles an hour.

Musk's plans for a "fifth mode" of transportation are undeniably wicked cool. The project is attractive in many ways: Building the Hyperloop would create jobs; the tube would be powered by solar panels, which would make it soft on the environment; and Musk's estimates peg the entire cost of the Hyperloop at less than a tenth of California's ongoing high-speed rail project, allowing ticket prices to theoretically be as cheap as $20 each way. But most awesomely: Passengers would zoom through a steel tube in tiny pods resting on a cushion of air.

"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," wrote Musk.

But like all bold plans (Musk's Tesla Motors and SpaceX companies included), the Hyperloop has attracted skeptics, some with nit-picky complaints, others with real questions that planners and engineers would have to face in making the project a reality. Among them:

1. Safety
Musk devoted a section of his plans to safety, including what would happen in the case of depressurization, especially important since the Hyperloop attains its ludicrously fast speeds by maintaining a low pressure environment within the tubes.

"[O]xygen masks would be deployed as in airplanes," he says. "Once the capsule reached the destination safely it would be removed from service. Safety of the onboard air supply in Hyperloop would be very similar to aircraft, and can take advantage of decades of development in similar systems."

But the Hyperloop would be intrinsically safe, says Musk. It would be "immune to wind, ice, fog, and rain," he says. "With human control error and unpredictable weather removed from the system, very few safety concerns remain."

"In many cases Hyperloop is intrinsically safer than airplanes, trains, or automobiles," he says.

But that's a tough call to make on an untested technology.

Such a novel system could attract terrorist attacks, Richard Muller, a physics professor at the UC Berkeley, told San Jose's Mercury News. And while the pods are blocked from wind, ice, fog and rain, Muller says, what about "more mundane threats, such as dirt and grime"?

And what would happen in the case of a power outage, or if cars get stranded between the cities while passengers are confined in their 27 inch-wide spaces?

"Engineers would need to determine exactly how the Hyperloop would deal with such contingencies if the system is ever to get up and running," says Mercury News.

2. Legal hurdles
The proposal says Hyperloop should run next to Interstate 5, a major California highway. Propped up on pylons, the system would disrupt a lot less land than a new highway or railway tracks, mitigating land-use conflicts, says Musk.

But the Hyperloop, which would need to be straight, wouldn't have the same flexibility to curve around restricted areas, including the miles of private farmland that runs along huge stretches of that highway.

Whoever builds the loop should expect a giant headache when it comes to land rights, said Bill Roberson in Digital Trends. "Expect the court bills to be more than $6 billion, let alone the $6 billion Elon says will be needed to build it."

3. Cost
Musk says the entire loop will cost between $6 billion and $10 billion to construct. Color most analysts incredulous. The bay bridge, which connects San Francisco to Oakland and can easily be traversed by foot, has cost $6.3 billion so far to renovate, Richard White, a professor of American history at Stanford, told The New York Times.

Tim Fernholz at Quartz explains why that $20 ticket price might be unrealistic as well.

Then there are Musk’s assumptions about ridership: He predicts 7.4 million people will ride yearly, allowing him to forecast one-way ticket costs at $20 plus operating costs if the capital investments are amortized over 20 years. But for that to work out, the system will need to average 30 full 28-person trips an hour, every hour of every day, with capsules leaving every thirty seconds during peak hours. Think that will happen in combination with airline-style security measures? Given fewer passengers and higher capital expenses, fares will be higher, too. [Quartz]

Musk and company still have a lot of planning and testing to do before the Hyperloop is taken seriously. But the entrepreneur says he's tempted to at least build a demonstration prototype to show it's possible.