A good-size sinkhole in downtown San Francisco wowed onlookers Wednesday as city crews got to work fixing the gaping crater that opened up during the evening commute the day before.

The 12-foot-by-5-foot pit on Mission Street between New Montgomery and Second streets was caused by a broken and aging sewer line — the third large San Francisco sinkhole to bust open in a little over a year.

Officials said it could take up to two days to fix the hole and get the streets reopened.

“Holy moly — this is insane,” Cupertino resident Ryan Lee said Wednesday while on his way to meet a friend for breakfast in the city. “It’s kind of scary, actually.”

Lee was one of scores of onlookers taking cell phone pictures of the growing spectacle that was first reported around 5 p.m. Tuesday when a sport utility vehicle got partially caught in the 9-foot-deep hole.

No one in the SUV was injured, and the motorist was able to drive away after getting a quick tow out of the pit.

As repairs are being made, 14-Mission buses are being rerouted to Market Street and drivers are being blocked from passing through the short section of street.

“This is one of the oldest pipes in the city,” said Jean Walsh, a spokeswoman for the San Francisco Public Utilities Commission. “But the bottom is still intact and the sewer is still flowing.”

The sewer main — 5 feet high and 3 feet wide — is one of the largest in the city and was built out of bricks in 1875. In recent days, some of those old bricks crumbled, and the earth and silt above funneled into the line and washed toward the city’s main treatment plant in the Bayview.

It took one unlucky driver to hit the flimsy asphalt and punch open the cavity.

That’s what happened last month when an even bigger sinkhole opened up in San Francisco’s Pacific Heights neighborhood. That one measured 22 feet by 17 feet and formed when an 18-inch sewer main broke under Sacramento Street between Lyon and Baker streets.

There are roughly 1,000 miles of such sewer pipes crisscrossing under San Francisco, and many of them are more than a century old, Walsh said. The city replaces about 15 miles of pipe a year.

One of the city’s more memorable recent sinkholes opened up in the intersection of Sixth Avenue and Lake Street in the Richmond District during a torrential storm in December 2014. Like the others, that hole was caused by an old, broken pipe, but was accelerated by the massive flow of water surging through the at-capacity main.

Tuesday and Wednesday were dry in San Francisco, so the only stuff moving through the pipe along Mission is what people were flushing, Walsh said.

Evan Sernoffsky is a San Francisco Chronicle staff writer. Email: esernoffsky@sfchronicle.com Twitter: @EvanSernoffsky