A city plagued by crumbling streets watched the mother of all potholes open up in one of its busiest downtown thoroughfares Friday — a hole stretching some 20 feet wide that could cost millions of dollars to repair and that provided instant fodder for amateur online comedians.

“This is nothing short of incredible,” said Mayor Mitch Landrieu, briefing reporters near the collapsed section of the 300 block of Canal Street between Canal Place and Harrah’s Casino.

No one was injured in the collapse.

The street had been blocked off earlier this month when engineers first started to fret about the integrity of a wall sealing off one end of a 50-year-old underground tunnel that was once envisioned as part of a riverfront expressway.

The collapse occurred about 3:30 p.m.

Engineers had been assessing the tunnel since Harrah’s first reported concerns April 19. Landrieu and members of his administration had gone out to view the site just hours before it eventually opened up, leaving a tangle of broken asphalt in a deep cavern in the left-hand lakebound lane of Canal.

The collapse also left portions of the neutral ground cracked and apparently at risk of collapsing as well.

City public works officials said they did not believe the hole would grow significantly, though damage was apparent on the streetcar tracks and the tunnel itself extends far beyond the area that collapsed.

Landrieu said the problem apparently started when water from a drainage line began leaking onto the steel-reinforced wooden wall that was built in the 1960s as a “temporary” closure to the tunnel, originally intended to be part of the controversial French Quarter riverfront expressway that was killed in 1969.

The leakage undermined the wall and caused it to buckle earlier this month.

Beyond the structural issues with the tunnel itself, the failure points to larger problems with infrastructure the city has long grappled with. After Hurricane Katrina, the city’s water lines were left seriously damaged, with about 40 percent of the water flowing through them leaking out into the ground.

Landrieu pointed to those problems as one of the causes of the latest collapse. The administration is working on a plan to use $2 billion from FEMA to fix some of those issues, as well as to repair the streets above the broken pipes that will be included in the project.

Director of Public Works Mark Jernigan said engineers have been evaluating the entire tunnel, including a similar wall at its Poydras Street end, and so far have no concerns about the rest of the structure.

The other walls and the ceiling of the tunnel are made of reinforced concrete and are designed to handle the weight of the roadways and buildings above them and the effects of the constant traffic in the area, Jernigan said.

Repairing the tunnel is expected to take between three and six months and could cost between $3 million and $5 million, said Cedric Grant, head of the Sewerage & Water Board and Landrieu’s chief infrastructure adviser.

It’s not clear where the money for the repair will come from. Several officials, including Landrieu, demurred when asked whether the city or Harrah’s or someone else would be responsible for picking up the tab.

That repair job could prove to be complicated. Rain could potentially undermine other subsurface areas that have not already collapsed, as was the case with a smaller sinkhole on Constantinople Street on Thursday.

“We’ll have a plan in place to minimize water getting in and pumps to get water out,” Jernigan said.

The tunnel is the only remnant of a plan by city officials to build an elevated highway between the French Quarter and the Mississippi River in the 1960s. That six-lane expressway would have been buried underground from Canal to Poydras streets to bypass traffic in the Central Business District.

In an attempt to jump-start the plan, city officials incorporated the tunnel, with a $1.3 million price tag, into the Rivergate Exhibition Hall built where Harrah’s now stands. But opposition from preservationists opposed to a highway alongside the Vieux Carre eventually led federal officials to scrap the project.

U.S. Secretary of Transportation John Volpe said at the time that “the public benefits from the proposed highway would not be enough to warrant damaging the treasured French Quarter.”

While never serving its intended purpose, the roadway was used first by the Rivergate and then by Harrah’s, which was built on the site in 1995. The two-story structure was used by the casino for offices and parking.

While the city blocked off the street above the buckling wall two weeks ago, it did little to provide updates on the status of the problem or the likelihood of an imminent collapse in an area that sees a high volume of foot traffic.

Repeated questions from The New Orleans Advocate about the assessments since the problem was first discovered went unanswered.

Thursday night — when cracks and settling were already visible from the street — city spokesman Hayne Rainey provided the first response to those questions with an email that said assessments of the tunnel and drain lines were still underway and that the closure of Canal Street was “out of an abundance of caution.”

The collapse provided some entertainment online Friday. A twitter account for the chasm was set up at @CanalStSinkhole and a satirical Craigslist ad offering up the cave as a New Orleans Jazz and Heritage Festival rental went online quickly.

“Be part of the resurgent New New Orleans in the new Sub South of Market neighborhood! Hunker down into the future like the rest of us natives,” the ad read.