A few hours before dawn on a recent Wednesday, a figure in black chest waders clambered out of a maintenance hole 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 maintenance hole to pull its heavy cover back in place.

Footage of the scene from August 18th was shared widely on social media and spawned rumours of an underground society as well as a debate about which cartoon characters the urban explorers resembled 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. (At around 1am on Thursday, a fifth group was captured on video entering a maintenance hole in Wantagh on Long Island, according to a spokesperson 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 such as the Teenage Mutant Ninja Turtles – some commenters, including retired transit officer Tony Hernandez, 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,” 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 spokesperson for the city’s Department of Environmental Protection.

The labyrinth of underground tubes contains “noxious gases, flooding risks and confined spaces”, Stewart added. “Members of the public should never enter a pipe, drain, catch basin, manhole or outfall.”

Phil Wong, a New York City Council member who represents parts of Queens, said in the past year his office has received several complaints of missing, dislodged or damaged maintenance hole covers. Maintenance holes provide entrances to both the city’s sewer system and some underground utilities.

The labyrinth of underground tubes contains ‘noxious gases, flooding risks and confined spaces’. Photograph: Jack Taylor/Getty ImagesThe labyrinth of underground tubes contains ‘noxious gases, flooding risks and confined spaces’. Photograph: Jack Taylor/Getty Images

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 maintenance hole covers, which can weigh more than 45kg, 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.

The episodes captured in May also came shortly after a summer storm – a perfect time to take the plunge, he said.

“When it rains, that has the effect of flushing out the nasty stuff,” added Duncan, who said he lowered himself into his first sewage maintenance hole in 2004 after tracking down 19th-century schematics of the system.

Duncan brushed off the idea that the groups may have been scavenging for items to sell. In his more than 100 trips into the depths of the conduits, he said the most valuable thing he found was an expired credit card.

“Out of all the ways to scavenge a bit of money in New York, buying headlamps and chest waders and going into a manhole cover seems like the most absurd way to go about it,” he said.

The first reported episode this year was on May 5th, around 2am, in Queens, when three men in chest waders walked toward an auto detailing shop in Astoria holding flashlights and a crowbar.

Admir Jakupovic, who owns the shop, Aki Auto Care, said he was working late inside the brightly lit garage when he saw the men.

“They looked like they were up to no good,” said Jakupovic, 34, who watched the men approach a maintenance hole on 20th Avenue.

He said he thought to himself, “Are you guys going to come in here or start something?”

He added, “We looked at each other for a quick second and then they just started opening the manhole.”

City officials later inspected the maintenance hole, where they found “no damage to the system,” Stewart said. Jakupovic, who stood by during the inspection, said he caught a nasty whiff of wastewater.

“It stinks down there,” he said. “Like, what were they doing there? It’s just weird.”

Two more episodes followed. On May 28th, at around 11pm, several people entered a maintenance hole in the Gravesend neighbourhood of Brooklyn, before climbing out three hours later, police said. The exit of some of the people was captured on video, and two of the men were later charged with criminal trespass based in part on a witness account, according to criminal complaints. They gave no indication of their motive, the police said, and could not be reached for comment.

At 1am, another group of people slid a maintenance hole cover open and entered the sewage system in the South Williamsburg neighbourhood of Brooklyn, police said. They emerged about three hours later and “fled the location in an unknown vehicle to parts unknown”, according to police.

Both incidents remain under investigation, police said.

Wong said his office has reported at least six exposed maintenance holes this month to the city, yet he said three still remained open.

Exposed maintenance holes can be perilous, he said. In May, a 56-year-old woman died after stepping out of her vehicle in midtown Manhattan and falling into an open maintenance hole.

There are also broader concerns about sabotage and infrastructure damage, Wong said. “If there are some ill-intentioned people that are looking for things to steal like copper cables or even looking for networks to disrupt,” he added, “then we have a problem here.”

This article originally appeared in the New York Times