SEATTLE — For an entire year, the world’s largest tunnel-boring machine has been stuck deep below this city’s waterfront. Engineers still don’t know why “Bertha,” the 326-foot long, 2,000-ton behemoth custom built to create a nearly two-mile long tunnel under downtown Seattle, isn’t working.

Bertha’s malfunction is delaying the most ambitious infrastructure project in the city’s recent history. After a 2001 earthquake damaged the Alaskan Way Viaduct, a two-level state highway that runs along Seattle’s waterfront, Washington state set out on a multi-billion dollar project to build a tunnel underneath its largest city, connecting an industrial area near major port facilities with transportation routes bypassing the crowded downtown streets.

But the city will have to wait. Chris Dixon, a top executive at Seattle Tunnel Partners, the contractors group picked to head the construction project, said last week that an effort to retrieve and fix Bertha will push the completion date back to August 2017 — nearly two years after the tunnel’s original expected debut date of December 2015. And even that projected delay may be optimistic.

Supporters of the tunnel project include Gov. Jay Inslee (D), Seattle Mayor Ed Murray, most of the city council and major businesses in the area that depend on the thoroughfare for transporting their goods into and out of one of the West Coast’s busiest ports. But even among their ranks, some are beginning to mention the tunnel project in the same breath as Boston’s Big Dig, the series of tunnels and bridges that took two decades to build at a cost nearly 10 times initial projections.

“Obviously, it’s very disappointing,” said Jean Godden, the Seattle City Council member who chairs the Central Waterfront, Seawall and Alaskan Way Viaduct Replacement Program committee. “We’re concerned about the disruption during the time that the tunnel construction is underway.”

Repairing the machine is itself becoming a major undertaking. Bertha, built by the Japanese company Hitachi Zosen Sakai Works especially for Seattle’s tunnel project, isn’t able to reverse itself. It has already tunneled a little over 1,000 feet into a more than 9,200-foot route, leaving engineers scrambling to fix it with two options: Burrowing into the machine from behind, or tunneling down from above and hauling the whole machine out. They opted to drill down from above.

The engineers have dug a 90-foot hole, three-quarters of the way to the machine. A massive red crane, which will eventually drag the malfunctioning front end of the machine to the surface, looms next to the Viaduct it will replace, towering above sound walls erected to block the hole from public view. But Bertha is stuck in soft ground just a few feet from the shoreline of Puget Sound, an area of the city that was itself once under water until engineers filled it in a century ago, said Joe Mahoney, a professor of construction engineering at the University of Washington. Freeing the machine from that soft ground has required pumping out surrounding groundwater to keep pressure off the hole.

That created its own problems last month when, in the course of removing groundwater, some buildings in the surrounding Pioneer Square neighborhood, the oldest in Seattle, sank about an inch, opening cracks in walls and frightening building owners and tenants. In an update last Monday, the Department of Transportation said it had surveyed 50 buildings in the historic district in the last two weeks. The same day, teams of surveyors were seen on neighboring blocks, measuring for any further potential settling.

Dixon, the project manager for Seattle Tunnel Partners, said they would continue digging toward Bertha, but only a few feet at a time, what he called a “very sequential basis,” to ensure no further damage to the local area.

STP anticipates repairs will be complete by April, when Bertha would be able to continue excavations for the first time in 16 months.

“The uncertainty is how long [repairs are] going to take, because we’re really attempting to do something here that hasn’t been done before,” Dixon said.

The decision to move forward on the tunnel came after a decade-long political fight. The state Department of Transportation initially decided, in 2004, to build a conventional “cut and cover” tunnel — digging a tunnel from above, like the Big Dig. Voters rejected that option three years later, along with another option to rebuild an elevated structure. Local and state officials chose the only remaining option, the bored tunnel, in 2009. That year, the legislature authorized $2.8 billion for the project.

Opponents, including some environmental activists, a few members of the city council, former one-term Mayor Mike McGinn — who lost his job to Murray in 2013 — and The Stranger, Seattle’s alternative weekly newspaper, have kept up a constant drumbeat of criticism ever since.

With costs spiraling as delays mount, some on the city council are nervous that Seattle could be stuck with the bill. And with Bertha sleeping, some tunnel opponents are reminding supporters of their pessimistic predictions.

“I think we’re at a point today looking forward where people need to be asking, okay, we’ve sunk a lot of money into this. If we get to a point where the contractor isn’t willing to sink any more money in, where do we go from there?” said city councilman Michael O’Brien, who opposed the tunnel when it first came up. “I hear a lot of frustration from folks that they’re not seeing elected officials step up and own up.”

Other observers already see signs that the Department of Transportation and the Seattle Tunnel Partners are preparing for lawsuits that are all but inevitable. The DOT is rigorously documenting every detail of the delay and the settling around Pioneer Square on its Web site. On a recent afternoon, survey crews were inspecting the area that had settled, and several blocks north, closer to Seattle’s downtown core.

Elected officials repeatedly invoke public safety concerns, which some observers say could indemnify the state against massive cost overruns in a legal case, should the tunnel be mothballed.

“It may be something that it will take some lawyers to figure out,” Godden said.

While Bertha remains dormant, STP has hastened construction of other parts of the project, like entrance and exit portals on each side, to compress the calendar as much as possible. WSDOT Secretary Lynn Peterson said this month the project is 70 percent complete, even though the boring machine has more than 8,000 feet to go. Matt Preedy, deputy administrator of the Alaskan Way Viaduct Replacement Program at the Washington State Department of Transportation, said that included segments that will eventually be placed within the length of the tunnel yet to be drilled.

But that isn’t what an increasingly frustrated public wants to hear. It “completely missed the mark from what I think the reality is, and what I think the public feels the reality is,” O’Brien said. “They may have spent 70 percent of the money.”

And until Bertha revs back to life, even STP’s August 2017 estimate is just a guess, based on assumptions that everything goes well in the future — something that hasn’t happened so far.

“Until the machine is underway and tested, the schedule should be expected to be dynamic, and since that repair work could take less time, it could take more time, we’re not in a position where we can say with any certainty how long that would take,” Preedy said.

The delay has given new life to criticism from tunnel opponents, some of whom are once again suggesting the state should abandon the project instead of throwing more good money after bad.

“If you’re a local tunnel supporter, this is a good time to be on vacation,” O’Brien said.