It started in early May. Under cover of darkness, three people pried open a manhole cover in Queens, New York, and clambered down into the sewer.

The incident might have gone unnoticed, but the subterranean quest, which was caught on film, captured New Yorkers’ interest when it happened again, and again, in the same month, with two other groups filmed making their way in and out of the sewer system in Brooklyn. The string of events have seen those involved dubbed “mole people” by the local press.

As the traditional news silly season kicks off, the mysterious sewer exploration has created summer intrigue in New York, with the city fascinated by a group of people who have, to date, been criticized by police, branded as “creeps” by a local tabloid newspaper, and compared, unfavorably, to a group of fictional crime-fighting turtles.

The first sewer episode happened on 5 May, at 2am. Three people, wearing hip waders and carrying flashlights, walked to a manhole cover in the middle of the road, hauled the circular cover aside, and clambered down into the darkness.

That was that, until Thursday 28 May, when a group of people shifted a manhole cover and climbed into the sewer in south Brooklyn. Hours after that, a group of people lowered themselves into a sewer hole in north Brooklyn.

“I could tell they were up to no good,” Aki Jakupovic, who witnessed the first event, told NBC New York. “They went in there, closed the cover, like, you know, they were never here.”

He added: “Three random guys walking around in a strange suit. Open the sewer, [and] go in like the Ninja Turtles.”

Jakupovic was referring to the comic book Teenage Mutant Ninja Turtles, which depicts the lives of four turtles who live in the New York City sewer system. The turtles, who are brothers, were exposed to radioactive waste as youths, which turned them into human-turtle hybrids. Trained in martial arts by a large rat, the turtles fight New York criminals. The story is not based on real events.

Still, the bizarre sewer escapades have captured the public’s attention. The New York Post, a rightwing tabloid, has been all over the story from the beginning, variously branding those involved as “weirdos” and “creeps”.

Other local news outlets have focused on what, exactly, people might be doing down there, with an emerging theory being that they might be scavenging for valuables in the city’s 7,400 miles of sewer pipes, accessible via 5,000 manholes. In 2015, a trainee at the New York department of environmental protection was arrested after being caught in the sewers, and last year three men were arrested after entering the sewer system in Brooklyn, allegedly in an attempt to find gold. They weren’t the first to have the idea – as far back as 1936 the New York Times reported that police had fished $3,500 from a sewer in Manhattan.

The New York police department told the Guardian there was “no threat to public safety at this time”, but said its intelligence division is investigating the sewer sojourns.

Rob Wolejsza, a spokesperson for the New York’s department of environmental protection, said in an email: “DEP has inspected the sewers at all locations, and our infrastructure is safe. DEP works closely with our public safety partners to help ensure the safety and security of critical infrastructure throughout New York City. Entering the sewer system is both illegal and extremely dangerous.

“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.”

Despite those risks, delving beneath New York’s streets is not a new phenomenon. The sociologist Terry Williams spent two decades visiting people who live in tunnels, hidden passageways and abandoned railroads, documenting their lives in the 2024 book Life Underground: Encounters with People Below the Streets of New York.

And New York isn’t the only place with a track record of people seeking refuge underground. Hundreds of people live in tunnels beneath Las Vegas, in passageways designed as storm drains to manage flash floods. Last year, Greater Good Charities, a non-profit, estimated that 1,500 people live in 600 miles of tunnels and culverts under the city.

As for the recent spate of sewer visits: people in the city have their eyes peeled for more footage of night-time explorers, even if they might not match up to Leonardo, Donatello, Raphael and Michelangelo.