A group of divers ventured to a remote sandy stretch at the outermost edge of New York City. Equipped with oxygen tanks, they entered the water, leaving behind the familiar urban landscape of concrete, traffic and littered streets. Descending into the ocean, they embraced a radically different environment, an underwater world.

Horseshoe crabs and other crustaceans crawl on a seabed encrusted with barnacles and colonies of coral. Spiny-finned sea robins, blackfish and wayward angelfish, swim in the murky ocean tinted green by sheets of algae.

Not all is pretty. Plastic bottles, candy wrappers and miles and miles of fishing line drift with the tides, endangering sea life.

The undersea litter isn’t always visible from the shore. But it has long been a concern of Nicole Zelek, a diving instructor who four years ago launched monthly cleanups at this small cove in the community of Far Rockaway, where New York City meets the Atlantic Ocean, about 4 miles (6.4 kilometers) south of John F. Kennedy International Airport in Queens.

A throwaway culture of single-use plastics and other hard-to-degrade materials has sullied the world’s waters over the decades, posing a danger to marine life such as seals and seabirds. By 2025, some 250 million tons (226.7 million metric tons) of plastic will have found its way into the oceans, according to the PADI AWARE Foundation, a conservation group sponsoring a global project called Dive Against Debris.

Dive by dive, small groups like Zelek’s have been trying to undo some of the damage.

"Every month, we have a prize for the weirdest find," she said. They have included the occasional goat skull, perhaps used as part of some ritual, Zelek surmises.

"The best find of all time was an actual ATM machine. Unfortunately, it was empty," she said.

The divers’ haul one late-summer Sunday wasn’t much, but there were clumps and clumps of fishing line untangled from underwater objects. What the divers can’t pull away by hand is cut with scissors.

"Unfortunately, tons of crabs and horseshoe crabs – which are under threat – get tangled in the fishing line, and then they die," Zelek said.

While more ambitious projects are underway to scoop up huge accumulations of floating debris in deeper waters, small-scale coastal cleanups like Zelek’s are an important part of the battle against ocean pollution, said Nick Mallos, vice president of conservation for Ocean Conservancy.

"The science is very clear and that’s to tackle our global plastic pollution crisis," he said. "We have to do it all."

Every September, the conservancy holds monthlong international coastal cleanups. Since its inception nearly four decades ago, the cleanups have retrieved about 400 million pounds (181.4 million kilograms) of trash from coastal areas around the world.

The best way to combat plastics going into the oceans, Mallos said, is to reduce the globe’s dependence on them, particularly in packaging consumer products. But human-powered cleanup is the least costly of all cleanup options.

The Dive Against Debris project invites what organizers call "citizen scientists" to survey their diving sites to help catalog the myriad of items that don’t belong in oceans, lakes and other bodies of water. By the group’s count, more than 90,000 participants have conducted more than 21,000 such surveys and removed 2.2 million pieces of junk, big and small.

Zelek and her fellow divers have contributed their findings to the project.

Surface trash might be easy enough to clear with a rake, but the task is more challenging beneath the water. Over the years, the layers of monofilament fishing lines have accumulated. And until a few years ago, no one was scooping out the line, hooks and lead weights.

Untangled, a pound of medium-weight fishing filament would stretch to a bit more than 4 miles (6.4 kilometers). It’s anybody’s guess how many miles of fishing line remain on the channel’s bottom.

"Those small things are really what starts to accumulate and become a much larger and bigger problem," said Tanasia Swift, who has been with the group for a year and works for an environmental nonprofit focused on restoring the health of New York City’s waters.

"If there’s anything that we see that doesn’t belong in the water, we take it out," she said.

While the drivers work, fishermen cast their lines from a ledge where the city’s concrete stops. The beach is frequented mostly by residents who live nearby.

Raquel Gonzalez is one such resident, and she’s been coming to the beach for years. She and a neighbor brought a rake with them on the same Sunday the divers were there.

"Needs a lot of cleanup here. There’s nobody that does any cleanup around here. We have to clean it up ourselves," she said.

"I love this spot; I love the scuba divers," Gonzalez said. "Look at all the good people here."