Mystery men who are “up to no good” have been roaming through New York City’s vast sewer system, and city officials on Tuesday warned potential copycats of dangers they’d face exploring Gotham’s treacherous underground.

There’ve been at least three documented sewer incursions since May 5 in America’s largest city, with no reported injuries so far, officials said.

“Entering the sewer system is both illegal and extremely dangerous,” a spokesperson for the city’s Department of Environmental Protection said in a statement Tuesday.

“Sewers can contain numerous hazards, including noxious and potentially deadly gases, unstable surfaces, flooding risks, and confined spaces. For these reasons, members of the public should never enter a pipe, drain, catch basin, manhole, or outfall.”

Auto repair shop owner Aki Jakupovic was working at 2 a.m. May 5 in the Astoria neighborhood of Queens when he made eye contact with sewer explorers as they lowered themselves.

“I knew they were up to no good,” Jakupovic told NBC News on Tuesday, recalling the strange encounter outside his shop. “They just looked up, look down and continued like I wasn’t there. You know, I’ve seen a lot of crazy stuff in New York, but this was really something.”

He and his staff called 911 and moved customers’ cars, parked outside, into the garage.

“I didn’t know if these guys were going to come up to me to try to start something,” Jakupovic said.

Responding officers told Jakupovic to do exactly what he did if he sees these men going underground again — don’t engage and call police.

“The cop was like, ‘Don’t act like Batman, like you’re not Batman, just call the cops,’” the night-owl mechanic said. “Like, yeah, that’s what we did.”

There’ve been at least two other New York City sewer incursions since Jakupovic’s encounter, both in Brooklyn last week.

Unauthorized entry into the sewers is a potential crime, though there have been no immediate arrests or evidence of a public health threat, officials said.

Investigators are pursuing their leading theory that the group is “scouring the system for valuables that get into the sewage,” a senior law enforcement official told NBC News on Monday.

“We need to know who they were, what they were up to,” said law enforcement analyst and retired NYPD Capt. John Monaghan, also warning the sewer visitors and their potential copycats.

“They could be electrocuted, they could be overcome, there’s gas lines down there, there’s electrical lines down there.”