Sounds great. But what’s it gonna cost?

Electric pods zooming through underground tunnels, ferrying passengers to and from the Loop to O’Hare Airport in 12 minutes — what’s not to like? It does sound sweet.

It could raise Chicago’s national profile tremendously. It could draw more corporate headquarters to town. Hot-shot CEOs really hate crawling through traffic or lumbering along on the Blue Line to get to O’Hare.

It could be a legitimate boon to local business, creating new jobs and generating more sorely needed tax revenue. Tourism might even get a boost, assuming folks from places like Japan and Denmark, if not Milwaukee or Cleveland, are prepared to fork over $25 for a quick zip into the Loop.

But what’s it gonna cost?

EDITORIAL

On Thursday, Mayor Rahm Emanuel put on a full-court press to sell Chicagoans on “Tesla in a tunnel,” which he and Elon Musk, the billionaire entrepreneur and Tesla founder, want to build in a quick three years.

They’re doing the big-vision thing, and there’s nothing wrong with that. We’ve never quite understood the thinking of those who say that going after the big stuff — a fancy park on the lake, a presidential center, an Olympics — necessarily comes at the expense of doing the everyday but crucial stuff, like upgrading public transit or improving schools.

But, really, what’s it gonna cost?

Because every promised freebie has always cost Chicagoans something.

The mayor and Musk insist that building and running the express service to O’Hare won’t cost taxpayers a penny. Tesla-in-a-tunnel, they say, will be built with $1 billion in private money. And Musk says he’ll recoup his entirely investment solely from advertising revenue and the fares riders pay.

If it doesn’t work out, he says, “me and others will lose a bunch of money.” And nobody else.

A question: Do you buy that?

Musk, who’s worth a reported $20 billion, can probably raise the money. And the city insists that its yet-to-be-hammered-out contract with Musk will lock in guarantees to ensure taxpayers face no risks. But real life doesn’t work like that. There is always a risk, and usually a price.  

Millennium Park was sold to Chicagoans as a privately funded venture but ended up costing taxpayers $270 million. The Obama Presidential Center has been sold as entirely privately funded, but already the city and the state are on the hook for an estimated $175 million for roadwork and related other costs.

We like Millennium Park. We’ll look forward to the Obama Center. And a 12-minute ride to O’Hare has its charms.

But, seriously, what’s it gonna cost?

The biggest risk with this project is that it will never be finished. The “Hyper-Loop” technology it would use is unproven. Even Musk admits that “often, things do not work” and that he chose Chicago as a proving ground.

So we’re the lab rat.

If this project goes sideways, what are the chances City Hall will allow its shiny new toy to sit half-done? Who will pay to complete it? And what’s the rush to begin construction in a mere three or four months when so many basic questions remain unanswered? We don’t even know the exact route the tunnel will take.

What will be above it? What will be below? Can we see an impact study or two?

As DePaul University expert Joe Schwieterman told Sun-Times reporter Fran Spielman, “God knows what you’ll find when you start digging through the city.”

Every detail of this proposed project should be spelled out and presented to the public. If done right, that won’t happen in just three or four months.

Send letters to letters@suntimes.com