NEW YORK, NY.- Adam Riback has spent a lifetime thinking about what lies beneath the surface.

Growing up in Sea Gate, a small community on the western tip of Coney Island in Brooklyn, Riback, 53, said he would spend hours gazing out at Gravesend Bay, thinking: “What’s underneath it? What’s down there?”

It wasn’t until decades later, when he happened upon a dive shop in Brooklyn, that he would find out.

Most New Yorkers probably don’t know it, but by some estimates there are about 5,000 shipwrecks scattered around the state’s shores, possibly one of the highest concentrations of wrecks in the world, according to one expert.

Since 1971, members of a New York City-based scuba diving club, Big Apple Divers, have been plunging into the coastal waters, uncovering shipwrecks and an array of aquatic creatures — sharks, whales, seahorses, ocean sunfish — hidden from plain sight.

Riback, now executive director of the New York State Marine Education Association, joined Big Apple Divers in 2011 and later served as the group’s vice president.

“Most people aren’t even aware that they could dive in the larger region, let alone close to Manhattan,” he said. “There’s a whole ecosystem in our backyard.”

Today, however, longtime members of Big Apple Divers fear that the local diving scene is dwindling. Older divers fret that they are being aged out and that a world of submerged history is being forgotten.

“The most common misperception is just that there’s nothing to see,” said Harris Moore, 34, who runs an introductory diving course for the club.

In what is known by divers as “Wreck Valley,” a triangle between the Jersey Shore and Long Island, divers can explore hundreds of shipwrecks, like the USS San Diego, near Fire Island. It was sunk by a German mine in 1918, the only major U.S. warship lost during World War I, according to the Naval History and Heritage Command.

Shipwrecks provide new habitats and hiding spaces for small fish, away from predators. In time, clusters of corals, anemones and barnacles blossom. Giant lobsters, bluefish, blackfish and striped bass waft among the crevices of sunken vessels.

“If you’re into the marine life, it’s like an underwater scavenger hunt,” Moore said. “You never know what you’ll find.”

In the past 50 years, storms have become increasingly frequent and ferocious, threatening to rip apart the sunken ships and the objects they contain. Within years, some may be destroyed entirely.

The dive chair of Big Apple Divers, Tracy Cloherty, said she had found buttons, bits of china and personal items from passenger bags during local shipwreck dives.

“We’re preserving this little bit of history,” Cloherty, 58, said. “I don’t know if anyone remembers these people or not, and people only exist as long as they remain in somebody’s memory.”

But diving is not for the fainthearted. The waters around New York are regarded as more “hard core” than somewhere like the Caribbean because of poor visibility, freezing temperatures and strong currents. Over the years, some divers have died during or shortly after explorations of wrecks. The Andrea Doria, off the coast of Nantucket, Massachusetts, is nicknamed the “Mount Everest of Wreck Diving” because navigating it is so difficult — and often deadly.

Those who are up for the challenge of local wreck diving describe submerging into muddy darkness — a foreboding green netherworld, sometimes so murky that they can’t see their fingers extended in front of them.

“They’re terrible days, where you just start feeling the dive site more than seeing it,” Moore said.

Jozef Koppelman, 66, who went to his first Big Apple Divers meeting when he was a teenager, said local divers have a sort of “selective amnesia”: One gleaming “postcard day” in New York’s waters overwrites nine when nothing could be seen at all.

“Part of the allure is the idea that it’s not consistent,” he said. “There’s a leap of faith with every trip.”

But Koppelman fears that things are changing. Many of his peers have retired from diving. Boats to dive sites have become less frequent.

Operating a dive boat or shop in New York may not be lucrative, given the limited clientele, the short dive seasons because of the weather and a shift toward buying equipment online.

Koppelman joked, “If you want to know what it feels like to own a boat, stand in a cold shower and rip up $100 bills.”

In recent years, a new generation of divers has been working to make New York’s underwater spaces cleaner and more accessible.

The Billion Oyster Project, a nonprofit group, works with over 100 New York City schools to restore the city’s oyster population.

These tiny creatures offer a range of environmental benefits, from filtering water to fostering biodiversity to softening the blow of waves during storms, standing as a buffer against flooding.

“We’re so disconnected from the harbor even though we’re New Yorkers and we live on this series of islands,” said Zoë Greenberg, 47, the project’s assistant dive safety officer. She trains students how to dive and build oyster nurseries in every borough, from Soundview Park in the Bronx to Lemon Creek Park on Staten Island.

She said that one of the best parts of her job is pushing back on the idea that New York water is dirty and uninhabitable.

“The water is now cleaner than it’s ever been,” Greenberg said. “It is safe to swim in most areas most days of the year.”

Nicole Zelek, 32, is the founder of SuperDive, which provides diving instruction. She hosts regular underwater cleanups in Far Rockaway, near Kennedy International Airport in Queens. Divers spot horseshoe crabs, sea robins and vibrant coral colonies.

“It’s magical that so much is going on beneath the surface,” she said.

But city beaches are also peppered with pollutants. Fishing lines trap crabs and other crustaceans. A shopping cart, an ATM and a row of bus seats have even been found in the waters, Zelek said.

“You see the impact of humans living next to the sea in a way that maybe you wouldn’t see in the Caribbean and a protected park,” she said. “It is a good place to remember our connection to the rest of the world.”

For Dr. Michael Rothschild, 62, a pediatric otolaryngologist and Big Apple Divers’ medical adviser, part of the magic of diving in New York is its equalizing effect.

“There’s really no different calculations for nitrogen loading for people who are Democrats or Republicans,” he joked.

Underwater, where language is reduced to a handful of gestures, social and political barriers dissipate.

“Most human beings have very similar wants and needs,” he said. “When you’re diving, everybody’s producing CO2. Everybody’s using oxygen. Everybody needs to breathe.”

This article originally appeared in The New York Times.