A torrent of water spotted spewing from the top of one of Philadelphia’s iconic skyscrapers Sunday drenched Center City sidewalks, confused onlookers, and set off a cascade of conjecture as to its cause.

Video of the mystery gusher spouting from the pyramid-shaped peak of One Liberty Place on Market and 16th Streets first appeared on Twitter about 7 a.m. Witnesses filmed the surging water for about five minutes before it came to an abrupt and just as inexplicable halt.

The lack of any alarm, fire or visible damage to the 61-floor, 945-foot structure at the time left bystanders scratching their heads. And initially, fire officials, the Department of Licenses and Inspections and the city water department said they had no explanations, either.

The building’s management did not return calls for comment and a security guard in the lobby declined to answer questions later in the day.

So, with no answers immediately available, social media swooped in to crack the case.

Some suggestions were more helpful than others.

Perhaps this was the return of the Curse of Billy Penn, some wondered — a reference to the old superstition among Philly sports fans that the designers of One Liberty had cursed the city’s teams with failure when they erected in 1987 what was then the tallest building in the city and the first to violate a long-held gentleman’s agreement that no structure in the skyline should exceed the height of the William Penn statue atop City Hall.

The Philadelphia Flyers’ googly-eyed, mayhem-loving mascot, Gritty, also emerged as an early suspect.

Others immediately noticed the stream’s unmistakable resemblance to a certain restroom activity.

And one football fan even chose to view the spewing spectacle as a good omen for the Eagles’ Sunday afternoon matchup against the Chicago Bears.

Several more serious commenters suggested that perhaps something had gone wrong with the building’s slosh tanks — massive containers of water that engineers often install on the top floors of skyscrapers to increase stability and to prevent them from swaying in the wind.

The only problem with that theory? Unlike its other compatriots in the skyline — such as the newly built, 1,121-foot Comcast Technology Center, which has five tanks holding 125,000 gallons of water in the ceiling of its 57th floor — One Liberty Place doesn’t use them.

It ultimately took nine hours after the cascading conundrum was first spotted for a solution to materialize.

Just after 4:30 p.m., L&I inspectors confirmed to Billy Penn that the torrent was caused by a test of One Liberty’s fire protection systems, which must be certified annually.

And although newer buildings are equipped with special drainage to prevent the water those systems produce from flooding the sidewalks below, One Liberty was built before such features became a requirement.

City officials, who were initially just as stumped as the rest of us, said that the building’s management should have taken more precautions.

“The certification company and Liberty One have been directed to get a street closure permit and police assistance going forward,” said Fire Department spokesperson Kathy Matheson in an email.

Staff writers Madeleine Ngo and Catherine Dunn contributed to this article.