The Boring Company scored a surprise victory in Chicago today with the announcement that Mayor Rahm Emanuel had selected Elon Musk‘s tunneling venture to design, build, and operate a rapid transit link between O’Hare International Airport and the city’s central business district. Even more surprising: Emanuel is adamant that the project will require no taxpayer money. The Boring Company has promised to pay for it.

The project will cost less than $1 billion, sources told the Chicago Tribune. But the civil engineers, infrastructure executives, and mass transit experts we spoke to are extremely skeptical of that figure.

Musk has said tunnels are the only way to "solve the problem of soul-destroying traffic.” And his promise to dig these tunnels faster and at a fraction of the cost of traditional methods is what has attracted so much municipal interest. The city of Chicago hasn’t released the bid, so it’s hard to say specifically what will happen if that estimate is too low. But we do know, generally, that the project will take longer and require more borrowing if the Boring Company’s projections are off base. That could mean more financial problems for Musk, more scrutiny for Emanuel and his administration, and another failed transportation project for the city of Chicago.

According to the Tribune:

In exchange for paying to build the new transit system, Boring would keep the revenue from the system’s transit fees and any money generated by advertisements, branding and in-vehicle sales, Rivkin and the company said. Ownership of the twin tunnels has not been determined, but the Emanuel administration plans to seek a long-term lease to Musk’s company, a source familiar with the proposal said.

Later in the article, Chicago Deputy Mayor Robert Rivkin is quoted saying the city will negotiate to ensure it “will share in any significant profits that are made” from the Boring Company’s project. Rivkin declined to offer a timeline for when the project might get built but said Boring was “very forward-leaning and optimistic about its timeline.”

At a press conference in Chicago on Thursday with Mayor Emanuel, Musk sounded optimistic about being able to cover the daily costs of running the rapid transit line, but he was more circumspect about the debts he was likely to accrue by building it. “I think it’s very unlikely that it wouldn’t be able to make up for its operational costs. Basically it’s certain to cover its operational costs.” He added, ”Whether it provides a good return on capital is a separate question.”

“if we fail, well, I guess me and others will lose a bunch of money.”

Asked what he personally gets out of the project, Musk said, “First of all, I should preface this by saying... what I would ask is that... this is a difficult thing that we’re doing. It’s a hard thing. It’s a new thing. And I’d hope that you would cheer us on for this. Because if we succeed, it’s going to be a great thing for the city. And if we fail, well, I guess me and others will lose a bunch of money.”

Consider the immense costs associated with transportation projects in the world, especially those that require tunnel boring: the Second Avenue Subway in New York City cost about $2.5 billion per mile, while the Line 14 Extension in Paris ran about $450 million per mile. The Boring Company claims it can dig 18 miles of tunnels for a fraction of those prices.

“What’s going to drive down costs below the subway in Paris by a factor of 10?” said Constantine Samaras, associate professor of civil and environmental engineering at Carnegie Mellon University. “I’m looking forward to finding out.“

Samaras noted that the Boring Company has been selected as what’s called a D-BOM contractor, which stands for design, build, finance, operate, and maintain. According to the Federal Highway Administration, a private entity in a D-BOM contract “is responsible for design and construction as well as long-term operation and/or maintenance services.” The public sector secures the project’s financing independently and retains the operating revenue risk. “So they’re on the hook to making sure this system performs well over time,” he said.

Others view these projections with more skepticism. Yonah Freemark, an urbanist and journalist who has worked in architecture, planning, and transportation, said he was shocked to read about the costs associated with Musk’s proposal.

“The first is that a construction cost of $500 million to $1 billion would already be remarkably low — effectively $29 to $59 million per mile of tunneling — if you exclude the cost of the stations themselves and the vehicles,” Freemark wrote in an email. “But the stations are likely to be expensive.”

The Boring Company has said it would rehabilitate the Chicago Transit Authority high-speed rail ”superstation” beneath Block 37, which was mothballed in 2008 due to $100 million in cost overruns and limited interest in private operator of the express service. That rehabilitation will cost at least $50 million, and building a massive new station under O’Hare would likely cost even more. The transit system’s O’Hare station will be located near the new global terminal Emanuel has announced as part of an $8.5 billion overhaul of the airport.

Then there are the vehicles, or “skates” as Musk calls them. The Boring Company says it will use modified Tesla Model X car chassis as an underpinning for its so-called Loop public transportation system. These vehicles would be transported on “autonomous electric skates” traveling at 125–150 mph. Electric skates will carry between eight and 16 passengers or a single-passenger vehicle, according to the company’s website.

According to the Tribune:

The Chicago system is expected to be able to handle nearly 2,000 passengers per direction per hour, with cars leaving every 30 seconds to two minutes, city officials said. How much a ride will cost is subject to final negotiations, but Boring has stated a goal of charging between $20 and $25 — or half the cost of a typical ride-share or cab ride to O’Hare, a source familiar with the talks said.

By comparison, New York’s Metropolitan Transportation Authority can squeeze about 2,000 people on a single subway train. That raises the question: why is the Boring Company limiting the size and capacity of its vehicles?

why is the Boring Company limiting size and capacity of its vehicles?

Freemark figures that Boring would need about 61 vehicles in an active fleet. Using some back-of-the-envelope calculations, he figures they’d want 75 vehicles total, with a few extras for backup. Buses are each a minimum of $300,000 to make; trains cost at least $2.7 million per car. So if Musk is making his vehicles at bus prices, that’s about $22 million for the fleet. And the fleet will likely need a maintenance facility, which will cost a minimum of $30 million, Freemark says. “Add those things together, and suddenly Musk is projecting only spending $348 to $848 million to build those 17 miles of tunnel — shockingly less than any similar project,” he says.

A more realistic estimate, Freemark says, is two stations for $100 million apiece, the vehicles for $500,000 each, and a maintenance facility for $50 million. “That would leave $213 to $713 million for the tunnels,” he says. “I’m quite skeptical, especially since to meet safety and ventilation requirements, this tunnel can’t just be bored straight from downtown. It will have to have emergency exists, exhaust valves, etc. throughout its route. Where will these go and what will their cost be? Given the very high frequency of vehicles Musk is proposing (30-second headways), these exits are essential.”

All that aside, an airport-to-downtown connection in Chicago is “a good way to demonstrate the approach and costs of the Boring Company,” Samaras said, “with a lot less risk than building out a multi-stop system with car elevators as shown in the Boring Company’s original pitch video.”

On its website, the Boring Company says it can lower the costs associated with tunneling through two methods: reducing the size of the tunnel diameter and increasing the efficiency of tunnel-boring machines. That includes using automation, increasing the amount of power to the boring machine, and replacing diesel fuel with electric power.

Our first glimpse of the Boring Company was from a short video tweeted by Musk in April 2017 that depicted individual vehicles being lowered below ground via car elevators that were seamlessly installed in the street. From there, it’s a high-speed ride through Musk’s tunnels on electric skates. After another short elevator ride, the car is back on street level.

The Chicago plan is certainly a deviation from that original vision, but large infrastructure products are difficult to do in the US. It took nearly 100 years to get any traction on the Second Avenue Subway in New York; across town, costs associated with the massive East Side Access project under Grand Central Station have ballooned to $12 billion. California’s bullet train project lumbers ahead, despite having no prospects for adequate funding to complete an initial segment with a chance of attracting riders.

“It is very difficult to build anything new in the US of scale,” Dean Wise, former vice president for network strategy at BNSF Railway, told The Verge. “And as someone who’s primarily privately funded, we have a billion dollars of hot money in our hand, ready to build some facilities on the West Coast. And we saw the five-year delays, the eight-year delay, and now nothing happens... When projects don’t get built at all, why should someone engage in that effort?”

So why is Musk engaging in that effort, when the possibility of failure is so high? At the press conference Thursday, the billionaire addressed it directly: “You know I do think that there is a role for doubters. People should question things, and it shouldn’t be taken as a given that things are going to work because often things do not work.”