Jamaica Bay, a massive body of water dotted with islands and marshes, is by far the largest open space anywhere in the five boroughs.

Although it is situated right next to the busy John F. Kennedy International Airport, most New Yorkers aren’t even aware of its existence – a fact that Dan Hendrick is out to change.

Hendrick is the writer and producer of a new documentary called “Saving Jamaica Bay,” which details the bay’s rich history, from an oystering outpost to its designation as a national park to its ongoing struggles with pollution and disappearing marshes.

“So few people know about Jamaica Bay, let alone know that it’s a national park, and in some ways we’re hoping the film can really do a little bit of soul searching among us New Yorkers,” Hendrick told City & State. “It’s the city’s largest open space by far – larger than three Prospect Parks, three Van Cortlandt Parks and three Central Parks, combined. It’s a huge piece of open space, but it’s not celebrated in the same way that the Statue of Liberty or any of the sites that we have here is, so we’re hoping to change that with the film.”

The documentary, which is narrated by actress and activist Susan Sarandon, will premiere in a sold-out showing at the Museum of the Moving Image on Thursday evening as part of the Queens World Film Festival.

Hendrick first came across the bay 15 years as a reporter in Queens covering the mysterious disappearance of salt marshes, and he got to know a community of people living on one of the islands. He has since written a book about Jamaica Bay and spent five years working on the documentary, his first.

Marshes play a critical role in protecting the mainland, Hendrick said, a fact highlighted during Superstorm Sandy. Since then there has been growing interest in the bay, but the loss of marshes continues even as some areas are restored.

“The marsh loss right now is estimated around 40 acres a year,” he said. “They’ve done a tremendous job of restoring some of the larger marshes. It costs a lot of money to do it, but you talk to people there, whether they’re environmentalists or local residents, they say it’s money well spent, because you’re protecting the environment and also you’re protecting a lot of these homes.”

But Hendrick said that more government resources are needed. Amid the natural beauty of the bay, its beaches and waters are still marred by litter, and graffiti is scrawled on park buildings.

“It raises some real questions about the level of funding that we’re putting in to care for Jamaica Bay,” he said. “This is our national park, our national heritage in a way, and definitely it’s challenging to be in the middle of a city, but we can definitely be doing a better job of protecting it.”