It doesn't hurt, getting punched in the face. Not like you'd think. It might sting a little when your face is cold and the first shot tears open the bridge of your nose. But after a couple of minutes, after you've been clocked a few times and your flesh starts to tenderize like a hammered chuck steak, after your brain catches up to the fact that you're in a brawl and dumps cortisol and adrenaline into your bloodstream, you will still feel the impact, but the pain — the pain doesn't register anymore. Even when a guy grabs the back of your head with his hands, hauling it down so he can launch knee after knee up into your face, it's not the blows that hurt so much. It's your pride. It's demoralizing to have a guy hang on your head like that, controlling you, making it impossible to stand up like a man.

Some shots do get through to you, of course. Like if you get tagged between your fourth and seventh ribs, up around the liver. A hard knuckle punch there can make your diaphragm spasm, so you can't catch your breath. That can buckle you in half. Make you sick to your stomach. Or if a guy's thumb gets by your orbital bone and booms against your eyeball. If it lands just right, the blow can expand the back of the eyeball like a squashed balloon and fracture the fine layer of bone inside the socket. That's a real motherfucker. The pain ripples straight into your brain.

But you don't have time to think about that, because you dropped your guard for a moment and now the guy's sitting on your chest and raining down shots, banging and banging, trying to get you to fade out. So you're feeling for some imbalance in his weight, some subtle shift amid the barrage that'll allow you to buck him off. And if you do manage to worm away, maybe you can take his arm with you, wrapping it up in your legs to hyperextend that elbow, try to rupture the joint capsule. If he doesn't tap out right away, you'll hear his ligaments tear and his bones creak. Or maybe you'll wrap your arm around his throat, lock his trachea into the crook of your elbow, and bear down so hard with your other arm that you close the huge carotid arteries in his neck. Unconsciousness comes fast if you do it right — 10 seconds, more or less. And if you've been there, you know it's mostly a feeling of being stuck, and being pissed at yourself that you didn't see the move coming. You feel your strength dim as he sinks that arm deeper into your throat and a kind of darkness falls over your mind.

"Find Dr. Shu!" Dana White yells into an iphone. "do not take Stephan to the hospital. We got to find Dr. Shu." White is president of the Ultimate Fighting Championship, better known simply as the UFC. He is a big guy, built solid from his days as an amateur boxer and gym owner, and as he listens on the phone he shakes his head and rubs one of his meaty hands over his smooth-shaved scalp. He hollers to an assistant in the next room, "Hey, Stephan Bonnar's got a big cut above his eye, all the way up his fucking head. Will somebody please get me Dr. Shu!"

Then White jumps to his feet, shakes my hand, and with a wide smile says, "Hey, what's up, brother? Good to meet you. Sorry I was on the phone. One of our fighters got cut. You got to get these guys to the right doctor or it can ruin a career."

White knows what he's talking about. He runs the hottest circuit in the hottest sport: mixed martial arts. It's known to the cognoscenti as MMA, and it has fast replaced the Sweet Science as America's favorite combat sport. When White took over in 2001, the UFC was near bankruptcy. Now its fights can fill a major arena 10 or 12 times a year, raking in $2 million to $4 million in ticket sales and, reportedly, as much as $30 million in pay-per-view revenues for each event. UFC's fight videos are among the best-selling sports DVDs on Amazon.com, and its Ultimate Fighter reality show is the most watched program on Spike TV. MMA even has its own clothing lines, including Warrior Wear and American Fighter (and, for the ladies, Fight Chix), ideal for making a nice middle-class kid feel like a badass.

The base appeal of the sport is obvious. Put a couple of guys trained in multiple fighting styles into an eight-sided cage known as the Octagon and see who comes out standing. We're talking about three five-minute rounds (five rounds for title fights) of the closest thing you'll find on TV to no-holds-barred all-out fighting. It taps into the fear and craving every American man has for mano a mano violence: the imposition of one man's will over another's, the assertion of status in the pack. Under White the sport strikes a careful balance between the respectability that keeps it legal and the bloody spectacle that keeps it satisfying.

White heads this growing empire from a stucco office building behind the In-N-Out Burger on Vegas's West Sahara Avenue. Inside his office he keeps the drapes drawn and the decor a kind of Gen-X fight-mogul chic, with black-and-whites of Ali, Bruce Lee collector's dolls ("given to me by the Bruce Lee family, actually; we're real tight"), and a framed skateboard from Tony Hawk ("a friend"). White speaks in bold declarations punctuated by profanity, and he jumps to his feet again to explain why MMA is enjoying such a surge in popularity.

"Fighting's primal," he says. "It's inside all of us. If you take four street corners on any intersection in any city on earth, with guys playing stickball on one corner, football on another, throwing dice on a third, and a fight breaks out on the fourth, which corner does everybody run to?"

But what about the fighters themselves, I ask. What kind of guy does this? And what's it really like?

White's answer is to lead me out of his office and down a couple of flights of stairs to the basement of the building, where UFC has a training gym filled with speed bags and heavy bags and teardrop bags for throwing knees and elbows.

"You ever done any jiujitsu?" White asks. "It'll fucking blow your mind. The first time I tried it, it was like taking the red pill in The Matrix. Hey, Mario!" White calls across the boxing ring to a huge Brazilian kid. "Come over here. This guy's a journalist. Roll around with him, okay? Show him what the ground game feels like."

"Now?" I ask. Not only is Mario big, he's a black belt.

White nods. "Take off your shoes and your glasses."

So I do, and Mario lies on his back and tells me to lean in between his knees, missionary-style, and try something.

"Like what?"

"Try to hit me," Mario says.

"No, don't do that," White says, clearly worried for me, not Mario. "Just try to choke him."

I try to press my forearm into Mario's trachea and he flips me around and locks my own trachea into the crook of his elbow, using his other arm to leverage the choke. My carotid arteries shut down, my blood pressure plunges, and my head feels like a swelling balloon. I remember the universal MMA way of conceding defeat: three taps of the hand on the other guy's body. Except I do more like 30, and they're more like slaps.

After Mario finally lets go, White moves the tutorial on to another aspect: the power of the kicks. "It feels like getting hit with a fucking baseball bat. You've got to experience it to believe it. In fact, let's have a guy kick you."

Turning to his PR director, White says, "Hey, Jen, take him over to the Xyience gym so Forrest can kick him in the leg, okay? But tell Forrest it's got to be just a little love tap. Seriously." White pauses to make sure she understands. "Really, Jen. That's important. You have to tell Forrest 'a little love tap.' He really hurt that last guy."