Red plastic party cups—popular at college parties—have been spotted being used for drainpipes at one station along San Francisco’s new $2 billion Central Subway line.

Riders took to social media to share photos and video of what appeared to be a classic Solo party cup spouting water during the subway's first weekend of service.

The cup seems to be directing water outward at the Union Square/Market Street station, retired architect and Save Muni member Howard Wong said.

Without the cup, "The water jet would expand the hole and could migrate in multi-directions,” Wong said, adding that the depth of the subway creates powerful pressure that pushes water out with great force.

"I’ve heard there is a lot of water down there and saw some of it for myself yesterday," Muni drivers' union rep Pete Wilson said. "It looks like the cup was a simple fix to keep the water off the wall."

Water intrusion is a serious issue for the new subway system. Persistent leaks delayed the opening, according to a monitor assigned to track the project for the federal government.

“Just hoping the station is still under warranty,” said Livable City‘s Tom Radulovich, who advocates for improved transit, streets and public spaces.

A Twitter user who initially reported the use of the Solo party cups subsequently tweeted, "Okay confirmed by MUNI: we ARE using party cups for drainage, sourced from Costco."

SFMTA spokesperson Stephen Chun said the cups are an "interim solution" to keep water off the wall and that they will be replaced with proper drain holes.

"We’re using the weekdays to continue critical operator training and are finalizing any changes or construction fixes that need to occur for January," Chun said. "We’re also collecting feedback we’ve learned from our riders during this soft launch period."