Bobby Gunn is pacing the concrete floor of an auto-body shop somewhere in the industrial badlands of a large northeastern city. “I’m worried,” he says, wringing his mallet-size hands. “I’ve heard this guy today is a head-butter.”

More pacing, more hand-wringing, Gunn sinks deeper into a momentary funk amid the garage’s taxidermy heads and faded American flags. Today’s opponent, a former Marine, has a reputation for dropping his head during matches, an old trick among gloveless fighters who can easily fracture a hand by striking the skull. Gunn, 42, has pulled the maneuver himself many times. He’s broken his right hand just as often. Another fracture could end his career.

“Everybody wants to take me down, brother, and make a name for himself,” he says, punching a wall of rubber tires. “I’ve gotta watch my back.”

Just when you think you’ve heard all the grizzled-fighter-taking-one-last-swing-at-redemption stories, along comes Gunn — the first bare-knuckle boxing champion the U.S. has seen in more than 120 years. Undefeated in 71 fights, Gunn rules the circuit, a nationwide underground network of pro boxers, mixed martial arts fighters, and accomplished street brawlers who enter the ring without gloves for as much as $50,000 cash. It’s dangerous and bloody and illegal almost everywhere. And if things go Gunn’s way — for once in his life — it just could be the next major fight sport.

The undefeated Bobby Gunn.

Back in 2011, Gunn and his team staged a legal fight, held on an Arizona Indian reservation. They expected around 50,000 to live-stream the online pay-per-view event, bringing in about $500,000. As it turned out, more than a million tried to watch, crashing the fight’s payment system, and with it, Gunn’s chance for a long-awaited payday.

Still, it was all the evidence Gunn needed that bare-knuckle fighting is ready for the mainstream, and he and his backers have spent the past five years trying to stage another aboveground bout. But it’s tough to overcome an outlaw reputation. In the aftermath of the 2011 fight, Tim Lueckenhoff, then head of the Association of Boxing Commissions, condemned the fight as “abhorrent, barbaric, egregious, in contravention of a multitude of federal, state, and tribal boxing laws and regulations, and, perhaps, criminal.” And most state boxing commissioners appear to agree.

But they haven’t counted on Gunn. Raised by nomadic gypsies on the icy shores of Niagara Falls, Ontario, he’s made his way with his two fists since leaving school after the second grade. Later, when his pro-boxing career fizzled, he began earning his keep in the quick-cash underworld, working asphalt jobs, and getting in daily gym sessions while raising a family. Now, as the battle-scarred face of bare knuckle, Gunn has more than 200,000 Twitter followers and something he has been fighting for almost his entire life: a shot at legitimacy.

Gunn’s opponent is lean, muscular, and looks to be about a decade younger. The two fighters — both in jeans and sneakers — circle one another. The Marine, an accomplished street fighter, seems jittery. He feints and jabs while dancing on his toes, exhaling loudly with each punch. A few land, but most don’t. Gunn is quick, feet planted firmly, dodging and weaving with his upper body, Gunn seems relaxed. He throws fewer punches — sharp jabs, mostly to the body — but his land. After just 90 seconds, he shoots a quick left hook to the stomach, a right hook to the kidney, and a devastating deep left hook straight to the heart. As his opponent doubles over, Gunn delivers a final jab, to the chin. The Marine goes down. “Get him up, get him up!” Gunn yells, jacked up now, marching back and forth. On the ground, the Marine shakes his head. Gunn helps him up. “Good punch, dog,” the guy says, and the men embrace. The fight is over. “I don’t know if you ever took a sledgehammer to the face,” the Marine later tells me. “But it was pretty equivalent to that. I’m actually surprised that my mouth is still moving.”

The crowd cheers. Gunn is $10,000 richer, but he looks like a guy who just missed his train. “I’m tired of fighting in the shadows like this,” he says, rubbing his knuckles while a mechanic slides the garage doors open and sunlight fills the room. “I want to make this sport legal.”

Walk through a chain-link-fence gate, down a narrow alleyway, past two snarling pit bulls, and through a metal door, and you’re in Ike and Randy’s Boxing Gym in Paterson, New Jersey. On a Tuesday in October, three weeks after his victory, Gunn is working a punching bag while his 19-year-old son, Bobby Jr., and Ossie Duran, the dreadlocked Ghanaian immigrant and middleweight fighter who manages the place, look on. “You smell that?” Duran asks, grinning. “That smells like sweat — it smells like war.”

Ike’s is a cramped, low-slung, windowless space with brick walls the color of smokers’ teeth, a full-size boxing ring, and a rack of sledgehammers for swinging at a six-foot tire. It’s located in the most crime-ridden neighborhood of Paterson, recently ranked as one of the most dangerous cities in the U.S. Once while training, Gunn heard gunshots outside, finished his workout, and then emerged to find police cars and the chalk outline of a body next to his truck. “If you want to fight monsters,” he says, pausing to catch the swinging bag, “you got to train with them.”

With close-cropped black hair and blue eyes, Gunn resembles a muscle-bound Baldwin brother more than a ruthless brawler. His bulk is packed within a 5-foot-11 frame, supported by the bowlegged gait of a cowboy. Every morning he wakes up at 6, shaves with a straight razor and rubbing alcohol, puts on his standard uniform — denim jacket, jeans, gray henley, black Pumas, cell-phone holster — and bangs out 200 pushups. He has no tattoos and does not drink. His favorite book is the Bible. “I’m a hugger,” he’ll say, nearly crushing your torso in a full embrace.

Gunn and his family live in a tidy apartment building nestled between a Home Depot and the New Jersey Turnpike. He’s been married to his wife, Rose, for 22 years. Bobby Jr., a rising junior middleweight gloved boxer, just graduated from high school; he lives at home and trains with his father. “My dad is involved in every aspect of my career,” Bobby Jr. says. “He’s my manager, trainer, promoter . . . everything.” In the mornings, Gunn drives his seven-year-old daughter, Charlene, to a nearby private school, her papaya-size French bulldog, Max, rides shotgun. Gunn loves watching her walk in wearing her uniform, just another normal suburban kid. “The other parents have no idea who I am,” he says, “and I don’t want them to.”

Charlene has reservations of her own. “Mommy gets upset, and it makes me upset, too,” Charlene tells me as we chat outside her school. “Sometimes he gets punched and sometimes he gets cut, but I know he’ll be safe.” Gunn doesn’t relish it either. “I don’t like the feeling,” he says about unleashing the “monster” in the ring. “I want to quickly get back to my family afterward and pretend it never happened.”

At Ike’s, Gunn is building a sweat. He lifts weights, jumps rope, and spars — along with more unusual bare-knuckle workouts like hoisting rope-tethered weights from his clenched teeth to strengthen his neck muscles and massaging a mixture of Epsom salts and rubbing alcohol onto his brow to tighten his skin against cuts. He exercises like he fights, in jeans and black sneakers with a styrofoam cup of Dunkin’ Donuts coffee at his side. No stretching. No water.

(A lifetime of fighting has left Gunn’s nose with no cartilage.)

Ike’s is more than just a gym for Gunn — it’s also his main connection to the underground fight circuit. He stops working the bag and points to a custard-colored landline phone the size of a toaster sitting on a metal desk. “This is where everyone knows to reach me for a fight,” he says. “I’ll come here to spar and see a note taped above the phone: ‘Yo, Gunn — call this number.’ ”

Bare-knuckle boxing thrives in the underworld. With no central governing body, the sport is run by local promoters who book fighters and charge up to $100 a man (it’s always all men) to watch, and bet, on a match. Those who wager on Gunn generally go home a little richer. “There’s only one champion of bare-knuckle boxing,” current world heavyweight boxing champion Tyson Fury recently said on a Twitter video, “and that’s Bobby Gunn.”

A lot of the men on the circuit are pro boxers and MMA fighters looking for extra cash. “It’s a different beast from boxing,” says Danny Batchelder, a pro who also fights in the underground. “It’s more pure. The politics ain’t in it, the crooked promoters and managers ain’t in it, there’s no corruption. It’s just who’s the better fighter that day.” There are no rounds, and most fights are brief, lasting between five and 10 minutes. Fighters wear street clothes rather than trunks, the better to blend in with the crowd should the cops bust in. The contest ends when a fighter is either knocked out or, more typically, says he’s had enough.

“Bare knuckle is like chess, and boxing is like checkers,” says Gunn. “If I put a glove on my hand, I can hit you as hard as I want, wherever I want. In bare knuckle, I have to hit you in the right areas or I’m going to break my hand. It’s slower than regular boxing.

You have to pick your shots.” Science suggests that it may be safer as well. A recent study found that while MMA fighters are more likely to get injured, boxers are more likely to suffer serious head trauma. Bare-knuckle fighters hit each other with less force than contestants do in either gloved sport. “Out of boxing, MMA, and bare knuckle,” says Randy Gordon, a former New York boxing commissioner and co-host of At the Fights on SiriusXM radio, “bare knuckle is the safest.”

Nonetheless, under existing law, organized fights without gloves are illegal. So the events come together clandestinely. For example, before one bout Gunn took me to, the fighters — one a Hells Angel and the other a former soldier — had been taunting each other via Facebook posts for several weeks. Offline, they agreed to stake $8,000 each, collected cash from backers, and then handed the “kick-in money” — a portion of the final prize money due on fight day — to a third party to hold. A location was established, but not revealed. Then, about a week before the match, the promoter texted a specific date and city to a trusted group of fans. The day before the bout, the start time was texted. The next morning, a text was sent with an intersection, which turned out to be the site of an after-hours gym in a hardscrabble section of a major East Coast city. The Hells Angel dropped the soldier to the floor in 30 seconds with a shot to the chin.

Critics knock bare knuckle as a fringe sport populated by undisciplined barroom brawlers who couldn’t hack it in prime time. It’s what they said about MMA some 20 years ago, when it was still outlawed in nearly all 50 states. Now MMA is a $600 million international success story. And not long ago, Ultimate Fighting Championship superstar Kimbo Slice tweeted to the world that he would fight Gunn in a sanctioned bare-knuckle bout later this year — a potential blockbuster event that would take the sport wide, finally giving Gunn a decent payday before his battered body breaks.

“I’ve only got another year or two to reign supreme,” Gunn says back at Ike’s, with the pit bulls barking outside.

Gunn has been brawling in one way or another almost since the day he was born. His parents were Travelers, the nomadic tribe mostly found throughout Ireland and the U.K. (Think Brad Pitt in Snatch.) An estimated 10,000 Travelers are said to live in the U.S., in secretive, highly insular communities. Deeply religious, Travelers segregate themselves according to Irish (Catholic) and Scottish (Protestant) origin, and they speak a language called Shelta — a mixture of Irish Gaelic, English, and a homegrown slang. (Gunn and his children regularly lower their voices and slip in and out of this language.) The men generally toil as laborers — paving, roofing, painting — traveling with the seasons in search of work. And when they have downtime, they fight. “If you have Traveler blood in you, you start going to the boxing gym at five years old,” says Mike Normile, a Traveler who has known Gunn since childhood and attends most of his fights. “This is a tough life, so you need to get used to dealing with hard stuff.”

The product of the rare union of a Catholic mother and a Protestant father, Gunn was born in Virginia but grew up shuttling between his father’s clan in Canada, and his mother’s in Tampa, Florida. But given his mixed heritage, he was largely shunned by both sides of his family. “On a campground, I was invited to a birthday party. My mother put a suit on me and I go up to their RV trailer and they hand me a paper plate outside. They wouldn’t let me in.” Gunn pauses. “That’s heartbreaking.”

The Gunns were poor, constantly moving from trailer parks to motels. “I remember sitting in hotel rooms and it’s 30 below outside, the wind blowing. We’d put a towel under the door to keep the heat in, and I’m sleeping on an old mattress on the floor,” Gunn says. Normile says his friend kept to himself. “Bob was a very small, very quiet little kid,” he recalls. “He was a bit of a loner and wasn’t getting on with different groups in the community — he had the weight of the world on his shoulders.”

When Gunn was eight, his father pulled him out of school and taught him to lay asphalt, paint houses, and work construction. But the real schooling took place after work. Descended from a long line of brawlers, Robert Williamson Gunn expected his only son to uphold the family legacy. “He was harder than coffin nails,” Gunn says. “My upbringing sounds cruel, but it gave me the will to never quit. He knew I needed to be an animal to survive.”

Gunn learned to control the tempo, keep his hands up high, and see the other guy’s mistakes before he makes them. He learned where to aim his shots. “Hit right underneath the heart and he drops to his knees,” Gunn says. “Hit him in the liver and it shuts him right down.” He rubbed a leather belt over his eyebrows to toughen them up and strengthened his knuckles by punching the wood floor. Sometimes his father would wrap a baseball bat with foam and duct tape and hit his son repeatedly in the midsection to harden his abs.

Gunn, it turned out, was a natural fighter. When he got older, his father would awaken him to fight men he had brought home from the bar. “He would say, ‘Can you beat that boy right there? How much do you want to bet?’ ” Gunn recalls. “I was 13 years old, fighting a full-grown man and knocking the piss out of him. It put them in shock to lose to a boy.”

Gunn began to formulate an escape plan. “It bothered me that I didn’t have things that a lot of kids had,” he says. “But I knew at age 13 that in order to have that stuff I would have to be the one to go out and get it — I’d have to be a hustler.”

(Taking a break with Bobby Jr.)

The hustle started in earnest in 1990, when Gunn was 16. His mother had grown severely ill and required a liver transplant. But due to her American birth and lack of proper marriage paperwork, she was ineligible for national health care in Canada. So Gunn moved to Las Vegas, hoping to earn enough in the ring to pay her bills. He soon met Carl King, stepson of promoter Don King. Gunn told King that he was 18, and King hired him as a sparring partner, promising to help him to the big time. Gunn moved to King’s training camp in Orwell, Ohio — a desolate facility in Amish country where he boxed from morning to night against men twice his age and weight, sending $500 a week to his mother. “These were middleweight champions of the world who were trying to kill me,” Gunn says. “At night I’d be pissing blood from the kidney shots. I woke up one morning and my pillow was all wet. My eardrum had busted, pouring blood. I lost 40 percent of my hearing.”

Gunn soon figured out that King had little interest in furthering his career. But he stayed for three years, until he’d sent home enough money to pay for his mother’s operation, which was unsuccessful. Gunn returned in time to see her die. “She had a massive heart attack while squealing in pain with a tube down her throat,” Gunn says, crying. “My mom never had no easy go.”

Disillusioned with the fight world and devastated by his mother’s death, Gunn quit boxing. He moved to a Traveler encampment in Fort Lauderdale, Florida, laid asphalt full-time, and married Rose, a beautiful blond Traveler he’d met years before. Rose hated fighting, and Gunn promised her that those days were through. It was a promise he did not keep. Gunn was invited to watch an underground bare-knuckle fight in a deserted warehouse in downtown Miami. The following night he returned and won two matches. He walked away with $1,500.

But it was about a lot more than money. Gunn’s boxing career was over before it had even begun. And now a new arena lay before him. Unbeknownst to Rose, Gunn was soon fighting in bare-knuckle matches up and down the East Coast, quickly rising to become one of the top fighters in the country. Even when bouts became “rough and tumble” — that is, all rules are thrown out and anything goes — Gunn never backed down. “Yes, I’ve bit ears off; I bit noses off, cheeks off, ripped open ball sacks,” he says. “I know that sounds graphic, but I’m doing it to some animal who’s trying to do it to me. I’ve had 300-pound gorillas on top of me, trying to smother and choke me out.” Once a Polish fighter tried to gouge out his eye. “He was a bad bastard who tried to do me dirt,” Gunn says. “So I put him to the ground and hit him with three left hooks. My thumb ripped the cartilage of the ear and it flipped over. I tore the side of his head off.”

Bare-knuckle fighting has always been bloody, but it wasn’t always so fringe. In the mid-19th century, it was the first nationwide commercialized sport in the United States. Today the hub of the sport is the New York City region, where Gunn and his family moved in 2001. He found work laying asphalt by day and trained when he could, taking on about five fights a year when the cash was right. But as his reputation spread, the phone at Ike’s started ringing. He fought on street corners in Latin Kings matches, in roadhouses for Hells Angels, in the backrooms of crowded Manhattan bars for the Irish mob, against one of Jay Z’s former bodyguards in a warehouse, and under subway overpasses amid syringes in events organized by the Jamaican Bloods. “It’s very scary at times,” says Normile, who has been at Gunn’s side for nearly all these bouts. “Ex-fighters, mafia guys, gamblers — I thought I was going to get killed at these events.”

Only once has Gunn shared that fear. It was at a fight organized by the Russian mob. Arriving late at night to a McMansion in the outer reaches of Brooklyn, he entered a large foyer to find an elegant party under way — men and women in eveningwear, live music, and a giant Russian man standing in the corner, waiting to fight him. “A hundred of these guys in suits and really weird Russian music playing,” Gunn says. “And this big hairy bastard, who’s 6-feet-5-inches and just wearing shorts. He looked like a water buffalo.” The crowd formed a circle, and the fight began. “He took awful shots — whack, whack, blood pouring out of him,” Gunn says. “As big as he was, he had no power.” Over the next 10 minutes, Gunn knocked his opponent to the ground five or six times, but the crowd would not let him quit. “They started screaming at him, kicking him to get up,” Gunn says. “They couldn’t accept it.”

Gunn finally knocked him unconscious, and all hell broke loose. “They’d put a lot of money on this guy,” Gunn says. One of the younger Russians pulled a gun on the fallen fighter and then turned it on Gunn. “This kid puts a big chrome thousand-dollar gun to my head,” Gunn says. “He was going to shoot me. Speaking his language, shaking that thing. The hatred boiled out of him, and that’s what makes you nervous, when someone is like that. ‘Oh, my God, this is a fool here. Please, God, get me out of here.’ ” Gunn remembers putting up his hands. “All I could think about was the last thing my wife and my little girl said to me that morning,” he says. “I never felt anything like that in my life. The blood rushing through my body, a gun to my head. I just said, ‘Please don’t.’ ”

After a tense standoff, Gunn was saved. “This old man in the back, the head guy of their organization, didn’t even get up from his seat. Calm, cool, and collected, he said first in Russian and then English, ‘Pay that boy his money. He’s a good man.’ So the kid put the gun down, they handed me a paper bag of money, and I walked out the door.” His payday: about $50,000.

But Gunn was growing tired of the bare-knuckle circuit. He had begun training his own son in gloved boxing and wanted to set an example. So in 2004, Gunn decided to mount a comeback. He was 30, the age at which most boxers begin to wash out, and hadn’t fought in a gloved match in years. He couldn’t afford to see a trainer or take time off to properly condition. He has no cartilage left at all in his nose — he can press it flat against his face like a rubber prosthetic. And he’d broken two bones in his back, fractured his elbow, and severely broken his right foot after falling from a two-story roof on a construction site in 2000. What’s more, he’d broken bones in his right hand countless times in bare-knuckle fights.

Nonetheless, he won 10 pro bouts, including the 2006 International Boxing Association world cruiserweight belt. In 2009, he scored the biggest match of his career, against the Polish cruiserweight Tomasz Adamek for the The Ring magazine world championship at the Prudential Center in New Jersey, in 2009.

On the day of the fight, Gunn drove his truck straight from a construction site to the arena. “I pull up in my work truck,” he says. “Smelling like an old dog, I get on the scale — not ripped, but in good shape. My opponent’s handler is reading contracts, signing things. They’re checking the gloves, and paint is coming off my hands. The guy looks at me like, ‘Where is this paint coming from? Where is my team?’ ” Gunn pauses. “No team. I fight, leave, and then go back to the job.” Gunn lost in four rounds.

In 2012, Gunn beefed up to a career-high 210 pounds but broke his right hand fighting former heavyweight world champion James Toney. After another loss in 2013, Gunn decided to hang up his gloves. “Nobody even talks about my boxing career no more,” he says. “But that’s what makes me better in bare knuckle. I’m not a barroom brawler — I’m an athlete.”

Gunn and Bobby Jr. stand on a windswept corner of the industrial docks overlooking New York Harbor in Bayonne, New Jersey. Their neon-yellow reflector jackets flap in the wind, while an industrial milling machine scrapes up the potholed cement like icing off a cake. In the wake of Hurricane Sandy, the port received a multi­million-dollar insurance claim, and the Gunns are working all hours for their piece of it. But not today. “Too windy to work,” Gunn says. “Would just get tar all over the truck.” Bobby Jr. nods, and they climb back into Gunn’s Dodge Ram.

Every morning after taking his daughter to school and finishing his first session at Ike’s, Gunn swings back home to grab his son for work. A muscular 154 pounds with freckles and a pompadour, Bobby Jr. is a lot like his old man. He tweets Bible verses to 6,000 followers, is unfailingly polite — and is also capable of dropping you like a wet sack of Idaho’s finest. “He looks like a wee boy,” Gunn says, “but God help the poor bastard that picks a fight with him.”

Bobby Jr. was destined to enter the fight world — but with a crucial twist. No bare knuckle. “Never done it and never will,” he says. It’s an ironclad rule set by his father. “I couldn’t bear for my boy to go through the violence I went through,” Gunn says. “So I give him what I could never have. I want him to be a top-class professional boxer.” So far, Bobby Jr. has a 5–0 record as a junior middleweight pro. Four of those fights he won by knockout. “I have a target on my head because my dad is Bobby Gunn,” Bobby Jr. says. “I’ve had to deal with it since my first fight ever.”

(Gunn drops his daughter off at school.)

As Bobby Jr. gets started in the ring, there are signs that his father could retire with the recognition he’s sought. In February, Gunn announced that after five years of scrambling, Bare Knuckle Fighting plans to hold a sanctioned bout in May, at an Indian reservation in the Florida Everglades, with Gunn as the marquee fighter. The pay-per-view event won’t be as pure as he would prefer — for safety reasons, fighters will wrap their wrists and hands with tape while still leaving the knuckles exposed — and Kimbo Slice wasn’t yet confirmed at press time, but it will be a giant step toward taking the sport out of the underground.

So far, the pro-boxing world is treating the new league as little more than a nuisance. But bare knuckle is seeing some support from respectable corners. Former boxing commissioner Randy Gordon believes the sport is almost certain to be embraced by hard-core fight fans. “Guys like me will buy every single pay-per-view,” Gordon says. “There’s room for boxing, MMA, and bare knuckle — they’re entirely different sports.” It certainly looked that way at the world heavyweight boxing championship in Brooklyn in January. After the fight, someone posted to YouTube a shaky 36-second smartphone clip of Gunn talking at ringside with Tyson Fury and boxing legend Mike Tyson. The three fighters are enjoying the undercard of a supertitle fight, a ring of paparazzi surrounding them. Yet the world knows only two of them.

Back at the port, as the late afternoon sun edges toward the horizon, Gunn pulls out of the construction site. Tonight there are tweets to post about an upcoming press conference in England, the postdinner regimen of 500 pushups, and, if the wind dies down, an asphalt job to finish at 3 am, when the docks are empty. In the meantime, there’s just enough of the day left for one last session at Ike’s. “It’s a bad world, my friend,” Gunn says, merging onto the turnpike, dust hanging like thin fog amid the mounds of asphalt, front-end loaders, and 18-wheelers. “What I’m trying to tell you is, thank God for my upbringing, my hard times. You see how I shine when I have to shine?”

As he drives, Gunn catches up on his Twitter messages, dictating his replies. He answers everyone, even the people asking for money or threatening his life. At a busy toll plaza near Jersey City, he rolls to a stop and fishes out some coins. “How are you doing, my angel?” he asks in his Traveler’s lilt. The toll-taker, a bored-looking middle-aged woman, smiles shyly. “Have a nice day, my darling,” Gunn says, driving away.

“People say, ‘Oh, my God, Bobby Gunn is ferocious,’ ” Gunn continues. “But I’m actually not the craziest person in the world. And I’d gladly do it all over again to give my children something better. At the end of the day, if you can’t do that for your family, then what does being a champion really mean?”