Adam Riback has spent his life thinking about what lies beneath the surface.

Riback, 53, grew up in Sea Gate, a small community on the western tip of Coney Island. He spent hours looking out over Gravesend Bay and wondering, “What’s underneath? What’s down there?”

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

Most New Yorkers probably don’t know it, but it’s estimated that there are about 5,000 shipwrecks scattered along the state’s coastline, which one expert says may be one of the highest concentrations of shipwrecks in the world.

Since 1971, New York-based diving club Big Apple Divers has been diving into coastal waters, discovering shipwrecks and a variety of aquatic creatures – sharks, whales, seahorses, sunfish – hidden from direct view.

Mr. Riback, now executive director of the New York State Marine Education Association, joined Big Apple Divers in 2011 and later became the group’s vice president. “Most people don’t even know they can dive in the greater region, let alone near Manhattan,” he said. “There’s a whole ecosystem in our backyard.”

Today, however, long-time members of Big Apple Divers fear the local diving scene is shrinking. Older divers worry they are getting too old and a world of lost history is being forgotten.

“The most common misconception is that there is simply nothing to see,” says Harris Moore, 34, who runs an introductory diving course for the club.

In what divers call “Wreck Valley,” the triangle of water between the New Jersey coast and Long Island, divers can explore hundreds of shipwrecks, such as the USS San Diego near Fire Island. Sunk by a German mine in 1918, it was the only major U.S. warship lost in World War I, according to the Naval History and Heritage Command.

Shipwrecks provide new habitats and hiding places for small fish, far from predators. Over time, corals, anemones and barnacles flourish here. Giant lobsters, bluefish, smallmouth bass and striped bass swim between the crevices of sunken ships.

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

Over the last 50 years, storms have become more frequent and more violent. They threaten to tear apart ships and the objects they carry. Within a few years, some of them could be completely destroyed.

Big Apple Divers dive leader Tracy Cloherty said she has found buttons, pieces of porcelain and personal items from passengers’ bags during dives to shipwrecks in the area.

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

But diving is not for the faint of heart. The waters around New York are considered “tougher” than the Caribbean, for example, because visibility is poor, temperatures are freezing, and currents are strong. Over the years, a number of divers have died during or shortly after exploring wrecks. One wreck, the Andrea Doria off the coast of Nantucket, Massachusetts, has been called the “Mount Everest of wreck diving” because navigation is so difficult – and often fatal.

Those who take on the challenge of wreck diving in their area describe plunging into muddy darkness – a menacing green underworld that is sometimes so murky they cannot see their outstretched fingers in front of them.

“These are terrible days when you feel the dive site more than you see it,” Moore said.

Jozef Koppelman, 66, attended his first Big Apple Divers meeting as a teenager. He said local divers suffer from a kind of “selective amnesia”: a brilliant “postcard day” in New York waters overwrites nine days of seeing nothing at all.

“Part of the appeal is the idea that it’s not permanent,” he said. “Every trip requires a leap of faith.”

But Mr Koppelman fears that things are changing. Many of his colleagues have given up diving. Boats to the dive sites are running less frequently.

With limited clientele, a short diving season due to the weather, and an increasing trend of purchasing equipment online, operating a dive boat or business in New York may not be profitable.

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

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

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

These tiny creatures provide a number of benefits to the environment: they filter water, promote biodiversity and dampen waves during storms by acting as a buffer against flooding.

“We are so far from the harbor, even though we are New Yorkers and live on this chain of islands,” says Zoë Greenberg, 47, the project’s assistant diving safety officer. She teaches students to dive and builds oyster farms in every borough, from Soundview Park in the Bronx to Lemon Creek Park on Staten Island.

One of the most rewarding aspects of her job is challenging the notion that New York’s water is dirty and uninhabitable. “The water is cleaner now than it has ever been,” Ms. Greenberg said. “Most days of the year, it is safe to swim in most areas.”

Nicole Zelek, 32, is the founder of SuperDive, a diving center. She regularly hosts underwater cleanups in Far Rockaway, near Kennedy International Airport in Queens. Divers discover horseshoe crabs, robins and colorful coral colonies. “It’s magical that so much is happening beneath the surface,” she says.

But the cities’ beaches are also full of pollutants. Crabs and other crustaceans get caught in fishing lines. A shopping cart, an ATM and a row of bus seats have even been found in the water, said Ms. Zelek.

“You see the impact of people’s lives by the sea in a way that you might not see in the Caribbean and in a protected park,” she said. “It’s a good place to remember our connection to the rest of the world.”

For Dr. Michael Rothschild, 62, a pediatric otolaryngologist and medical adviser to Big Apple Divers, part of the magic of diving in New York is its equalizing effect. “There really are no different calculations for nitrogen exposure for Democrats and Republicans,” he joked.

Underwater, where language is reduced to a handful of gestures, social and political barriers dissolve. “Most people have very similar desires and needs,” he said. “When diving, everyone produces CO₂. Everyone uses oxygen. Everyone has to breathe.”