(CNN) -- Call it the "cheese dip."
A 400-foot section of the Leo Frigo Bridge -- named after a late Wisconsin cheese scion -- suddenly sagged Wednesday, forcing police to close off the span that carries Interstate 43 over the Fox River some 120 feet below.
"There's a part that's sagging," a motorist told an incredulous 911 operator, according to recordings posted by CNN affiliate WLUK.
"A part that's sagging?" the operator asked.
"Instead of a bump, it's a dip," the driver said.
It appears that one of the piers holding up the bridge sank about 2 feet into the ground, Gov. Scott Walker told reporters Wednesday.
The bridge, which carries 40,000 cars a day, will be closed indefinitely, state officials said.
"We understand the disruption this is going to cause to traffic," Wisconsin Transportation Secretary Mark Gottlieb told reporters Wednesday.
The bridge was built in 1980 and last inspected in August 2012, declared sound and renovated shortly thereafter, said Kim Rudat, regional communications manager for the Wisconsin Department of Transportation.
The closure comes as an unwelcome addition to the city's traffic headaches, motorist Dennis Kelln of Green Bay told WLUK.
"It's a big hassle, because of all the construction that's going around town," he said. "The downtown bridge was closed for how long? Now this one is out? It's just like it's a never-ending problem here."
It could be worse, of course.
State officials say the bridge is in no danger of collapsing like the Interstate 5 bridge over the Skagit River in Mount Vernon, Washington, this spring.
In that incident, a tractor-trailer struck a support on the bridge, which had been rated "functionally obsolete," sending several cars plunging into the river below. No one died, but three people were taken to the hospital for treatment.
The Wisconsin bridge is named after Leo Frigo, who ran Frigo Cheese Corp. until his retirement in 1983. A former member of Roman Catholic religious orders, he was well-known in the area for his work with ex-convicts and for founding a Green Bay food pantry, according to his official biography on the pantry's website. He died in 2001.