Some tourists decided to go for a swim in famed Saint Mark’s Square, in front of the city’s cathedral.

The cathedral itself was damaged by flooding as water submerged part of the floor in the central part of the basilica for only the fifth recorded time in its nine-century history, officials said. The water covered “several dozens of square meters” of the marble pavement in front of the altar of the Madonna Nicopeia, a 12th century icon, and submerged the baptistery, the board responsible for the building said in a statement.

Near the covered entrance to the basilica, the mosaic floor was under as much as 35 inches of water, it said, “soaking the monumental bronze doors, columns and marble.” Water levels remained above ground in the basilica for 16 hours.

“It may not be visible to the eye, but structures age because of the salt water drenching the bricks, which were not meant to remain underwater for long; that goes for bronze, too,” said Pierpaolo Campostrini, one of the board members. “The bricks are like sponges, and if the water levels don’t drop, the water rises several meters to the mosaic level.”

“In one day, the basilica aged 20 years,” he said.

An editorial on Tuesday in the Venice daily Il Gazzettino asked what had happened to the Moses Project, the divisive, still-unfinished, multibillion-dollar system of floodgates that has been under construction for years. Venice, built on a lagoon of the Adriatic on Italy’s northeastern coast, has always been vulnerable to flooding, and the system of barriers is supposed to offer some protection as global warming and rising seas make the threat worse.