A few hours before dawn on a recent Wednesday, a figure in black chest waders clambered out of a manhole in Midtown Manhattan and slowly walked away.
Moments later, a second person in waders and a headlamp emerged from the same sidewalk sewer on Seventh Avenue across from Pennsylvania Station. Then a third person popped out, sat down and straddled the open manhole to pull its heavy cover back in place.
Footage of the scene from Aug. 18 was shared widely on social media and spawned rumors of an underground society as well as a debate about which cartoon characters the urban explorers resemble most: the Minions from the “Despicable Me” movies or the Mario Brothers of video game fame.
This was the fourth reported episode since May in which people have been recorded venturing in and out of maintenance holes in the city’s vast sewer system, according to witnesses and city officials. (Around 1 a.m. Thursday, a fifth group was captured on video entering a manhole in Wantagh on Long Island, according to a spokeswoman for the Nassau County Police Department.)
In July, officers arrested two men who they said were sewer diving in Brooklyn and charged them with criminal trespass. But the motives of those who have descended into the 7,500-mile network of pipes have not yet been determined, the police said. Theories range from scavenging for valuables to undertaking risky urban exploration.
Amid the online wisecracks — and frequent references to other subterranean dwellers like the Teenage Mutant Ninja Turtles — some commenters like Tony Hernandez, a retired transit officer, raised alarm about the sophistication of those who surfaced this month.
“The way that last guy popped out was too smooth, definitely not his first time,” Mr. Hernandez said in a comment on a video of the men emerging near Penn Station.
City officials have warned against more copycats, calling the conduct dangerous and illegal. It is also a threat to the “safety and security of our critical infrastructure,” said Déja Stewart, a spokeswoman for the city’s Department of Environmental Protection.
The labyrinth of underground tubes contains “noxious gases, flooding risks and confined spaces,” Ms. Stewart added. “Members of the public should never enter a pipe, drain, catch basin, manhole or outfall.”
Phil Wong, a city councilman who represents parts of Queens, said in the last year his office has received several complaints of missing, dislodged or damaged manhole covers. Manholes provide entrances to both the city’s sewer system and some underground utilities.
Mr. Wong said he spoke with cable workers in his district who told him that scavengers may head underground to snatch old copper wires.
Some of the manhole covers, which can weigh more than 100 pounds, may have been removed, he said. He said he planned to push for a hearing to discuss further securing or sealing the manholes.
Steve Duncan, who spent two decades urban exploring in New York City, thinks the sewer divers show signs of being experienced underground adventurers. They wear head lamps, have the know-how to remove covers and travel at night, when they are less likely to be seen and wastewater is typically low.
