Just how powerful is the legacy of “Jaws,” the original summer blockbuster, which turns 40 this week? It’s certainly responsible for generations of nervous beachgoers eyeing the black depths of the water with fear of what monsters lurk below.

It’s so powerful that it even made phobia experts — the people whose job it is to talk people down from their irrational fears — afraid to take a dip in the ocean.

That’s the case for Ali Mattu and James Hambrick, both senior clinical psychologists at the Columbia University Clinic for Anxiety and Related Disorders. Their professional training tells them shark phobia is about as sensible as fearing a T. rex attack in Manhattan — and yet they’ve both grappled with their fear of fins since seeing “Jaws” as kids.

“ ‘Jaws’ was a source of my own shark phobia,” says Hambrick, 41, who was born in West Virginia and has been with the clinic for a decade. “I’m pretty sure I saw it at home on TV the first time. I remember watching most of it from behind a couch . . . I was pretty freaked out at the time.”

Mattu first saw the movie when he was about 7 years old, young enough to create a tide of fear of the beasts that washed into his life way past the beach. Not only did he avoid deep water near his home in Northern California, he stayed clear of the shallows — and even the water in his bathtub.

“I thought there was open water in the bathtub,” he says. To avoid a drain-based shark attack, he’d take showers by standing on the edge of the tub. “[My parents] hit me over the head and said, ‘What are you doing?’ I said, ‘It’s in case Jaws comes!’ ”

Both psychologists are hardly alone in their lingering galeophobia (fear of sharks) and thalassophobia (fear of the open sea) that the Steven Spielberg film sewed into the lining of the world’s swimsuits in the summer of 1975. Especially when you consider the main antagonist of the film, the infamously malfunctioning rubber shark Spielberg built, is only on screen for a grand total of about four minutes.

In a time of CGI-created genetically modified megadinosaurs (like in “Jurassic World”) and intricately rendered fantasy worlds (basically every other movie), the simple tale of three men hunting a shark on a boat now seems like a no-budget student film.

Still, it has endured enough that it’s returning to the big screen: Select movie theaters nationwide are hosting two-day-only showings of the movie on Sunday and Wednesday.

So what is it about the film — made for an estimated $8 million — that still colors people’s nightmares, even during a time when sharks are propelled by preposterous tornadoes on cable TV?

Hambrick says your “Jaws”-related shark terror is simply evolution pulling strings on your brain — the same fight-or-flight response that could have saved your ancestors’ life out in the African savanna.

“When you go out into the water, there’s this idea you’re incredibly vulnerable,” he says. “Literally anything can kind of happen. We’re built to kind of fear that, we’re built to fear the unknown.”

Hambrick suggests thinking of early humans hearing a rustling in a brush or feeling something slimy slide by their legs in the water: Your brain tells you to run because your odds of survival are better if you assume that’s something coming to eat you instead of a friend coming to say “hi.”

“It really came down to our ability to think through faster and get away,” he says.

Unlike CGI monsters, which can look flat and cartoony, the mechanical shark in “Jaws” has to abide by real-world physics — making the attacks, like when shark-hunter Quint gets chomped off the boat, seem like real-world threats.

The taut tension of the film, aided by Spielberg’s economical use of the actual shark on screen combined with John Williams’ iconic, haunting score, plucks fear strings so effectively, it even transfers completely off-screen.

Claire Gresham found that out last week when her acting troupe, Ten Bones Theatre Company, did a live performance of “Jaws,” entirely from memory, at Videology, a bar and screening room in Williamsburg, Brooklyn. Even though it was meant to be a goofy comedy performance, the audience actually gasped when the shark (another actor) jumped out from under a table to attack Gresham, who was playing the swimmer who’s attacked in the opening scene.

“All of the attack moments sort of make up the structure of it when you’re remembering it,” says the 29-year-old Ditmas Park, Brooklyn, resident, adding that she was “traumatized” by the movie when she watched it again to prepare for the show. That trauma was a hangover from the first time she saw it as a little kid, triggering her own shark phobia.

“It’s hard to watch, because it’s really scary,” she says.

More than any piece of pop culture, “Jaws” ignited a specific and lasting fear (monkey houses at zoos didn’t exactly go out of business after “King Kong” came out).

The problem, however, is that the fear is entirely disproportionate to the reality: The chances of dying in a shark attack are just one in 3.7 million, according to National Geographic. The real dangers of the world are gas-powered: You have a 1 in 112 chance of dying in a car accident in your lifetime, reports the National Safety Council. The frenzy over shark fears got so bad that Peter Benchley, who wrote the book “Jaws” is based on, later said he regretted depicting sharks as killing machines.

The first movie Joe Romeiro ever saw was “Jaws,” and it sparked in him a terror that bled into eventual fascination and respect. He went from pretending he was shark-diving, by jumping off his couch into a pile of stuffed sharks, to doing the real thing: He’s been doing professional shark filmography for 10 years, and will appear on a handful of shows on the Discovery Channel during this year’s Shark Week, which starts July 5. The New Hampshire man quickly learned that the “Jaws” shark was nothing like the real thing.

“There’s never been a shark that’s outright been aggressive to me, where I felt like it was aggressive to me to hurt me,” says Romeiro, 39, who was also featured in the 2012 Shark Week Show “How Jaws Changed the World.”

“In reality, we all know that we’re not on the menu. Their biggest dream is to just have a day not bothered by anything.”

Mattu eventually got over his fear in time for a trip to Hawaii, through the same thing he and fellow psychologists recommend for their phobia patients: gradual exposure therapy.

“You don’t want to be that anxiety disorder therapist who never overcame his fear of sharks,” he jokes.

That therapy means easing people into situations they’re afraid of — by visiting sharks in an aquarium or wading in shallow ocean waters. But the problem with shark fears — versus fear of other things like lightning or spiders — is that most people don’t encounter them on a regular basis. The fear of what lurks below may forever be tied to Spielberg’s film and that ominous fin poking out of the water.

“Everyone goes out to the beach in the summer,” Mattu says. “The film keeps hitting you with that uncertainty.”