“I’ve been bitten thousands of times,” offers a chuckling Jeremiah Sullivan. “Been thrown around a bit. Beaten up pretty good. Nearly had my teeth knocked out. Certainly chewed on a lot.”

He’s talking about sharks.

Since the early ‘70s, Sullivan has been the world’s leading chronicler of human-shark interactions. A bearded, shredded-up marine biologist in his sixties, he’s taken thousands of dives, and been intentionally bitten by sharks more times than he can count in order to test his custom-designed suits of armor to protect humans from shark bites.

“We’re studying human-shark interactions, and the only ones I’m interested in are those that are most accurate and realistic so that we can protect anyone who happens to be in the sea and learn the behavioral range and capabilities of these apex predators so we can conduct safer interactions,” he tells me.

On the evening of July 17th, National Geographic will air the special Man vs. Shark, featuring Sullivan being intentionally bitten by a deadly, 14-foot tiger shark in order to test his latest protective gear—a composite suit built to withstand the blow from an ax. Given its saw-shaped teeth, so sharp they can easily mash through turtle shell and bone, even Sullivan found himself a tad bit nervous about the risky stunt.

“I felt pretty confident in what I was doing but the tiger sharks I’d been saving for later, they’re known to have among the most destructive bites and to do a lot of damage when they get ahold of things and try to chew on them for a bit,” he says. “We weren’t sure what was going to happen. I had a lot of people with me that were quite sure that when one bit me, the other tiger sharks were gonna come swarm on me.”

He laughs: “I had to approach this thing knowing that it could be bad, but I was sure I was on the right track.”

Sullivan’s fascination with sharks began early on. He grew up in Hawaii (“before it was state”) and Puerto Rico, where he was raised by his mother, who worked for the World Health Organization (his father, a Hollywood actor of the same name, was largely out of the picture). In addition to his proximity to sharks and access to the sea, one of his biggest inspirations came in the form of the film Born Free, a 1966 picture telling the tale of a couple raising orphan lions in Africa.

“It occurred to me, wow, if we can have these friendly encounters with these so-called worst-of-the-worst predators, then why would that be limited to one or two species?” he recalls. “Why wouldn’t these animals have the same range of capabilities that other species have? There could be more to the story than the hyperbolic noise we’re hearing out of the media of ‘shark attack this,’ ‘shark attack that.’”

While attending Scripps Institution of Oceanography in San Diego, he came across a research paper suggesting that sharks were repelled by the galvanic currents generated by metal in seawater. So, in the ‘70s—prior to the release of Jaws, he notes—Sullivan began developing shark suits made of chainmail that could withstand bites from sharks under 9 feet in length. The results were remarkable.

“Sharks 98 percent of the time are completely chill with us under normal circumstances,” he says, adding, “I’m not a big fan of people that don’t demonstrate, through their performance, the appropriate respect for these kinds of animals. When someone does get bit it’s frequently through stupid human tricks, like trying to show off.”

Sullivan appears uniquely attuned to the rhythms of sharks. Three decades ago, after shooting the TV docuseries Wild Kingdom in South Australia, he was on a shark dive with a bunch of pals when something extraordinary happened: he exited the cage and hitched a ride on the tail of a 17-foot great white. The moment, which was captured on film and is included in Man vs. Shark, was the first friendly encounter documented between a human and a great white shark.

“It became apparent to me that the shark was so chill that I was drawn out,” remembers Sullivan. “So I slipped my hand under its caudal fin as it swam by, and the shark was so big it didn’t even flinch. You know, it was important to me to make very clear to the powers that be out there that kept putting out all this hostile conversation about these animals that that wasn’t the whole story.”

There are several moments during Man vs. Shark that are sure to raise one’s temperature. During one dive, Sullivan—donning a sleeveless vest—and his team descend into heavily shark-infested waters. A storm begins to form up above, making the water murky and difficult to spot sharks. Out of nowhere emerges a giant hammerhead shark who charges him; he gracefully redirects it with his arm. Then, a 7-foot bull shark (third-deadliest) brushes his body—a mere amuse-bouche to the big ol’ tiger shark lying in its wake, who attempts to push past him. Again, Sullivan casually guides it away with his exposed arm.

“It can get quite challenging and be extremely dangerous,” he says of the encounter, emitting his trademark chuckle. “But you know, I’m always looking to confirm my belief that there can be much more to the interactions between humans and sharks in the sea, and I’m delighted to say that that’s proven out to be true.”

As for the big “tiger-shark bite” moment in Man vs. Shark, well, it turns out to be not one, but three big bites. We see Sullivan approaching a group of tiger sharks with his armor-suited arm extended, which he built in a way where he could pull his arm out of the test component “should things get out of control.” The suits, he says, uses “a variety of advanced materials” to “interrupt the process of trying to evaluate whether you are something bitable and eatable [to a shark], or just something that should be run off and scared away.”

The tiger shark clamps down on his arm with 400-plus pounds of force but only manages to give the armor a few nicks and scratches. Then he extends his arm out again for another bite. And then a third one, wherein the shark drags Sullivan for a few feet before releasing him.

“They become far more powerful in their strength than their bite,” he explains, with bones easily snapping under the pressure. “So the new suits are all about responding not just to the bite pressure but to their force. We’re addressing the kind of damage that can happen to the human body—with me being the guinea pig, here.”

So is Sullivan, whose mates call him “Iron Man,” fearless?

“I’m not fearless at all,” he says. “Fear is a funny thing. Fear of the unknown is really all it comes down to, so if you’re well-informed about the environments you’re going into, and prepared as can be, it puts you in a pretty good position. Being a water guy and in the ocean virtually my whole life, it’s not something that’s frightening or fearful to me. It’s more like home.”