Elon Musk is in the news. (Again.) The latest announcement came last Thursday when Musk’s Boring Company signed a contract with the city of Chicago to build an “express loop” from O’Hare Airport to the city’s downtown. In one important way, the deal is wholly unlike most of Musk’s other projects—it won’t be taxpayer subsidized.

In other ways it’s exactly like his other schemes: Musk claims that his engineers can create an 18-mile underground passage for $1 billion—one-twelfth of the typical costs for such a project. Musk insists that he’ll employ a revolutionary technology to achieve the savings. And the approach he’s proposing has never been tested: