PACIFIC NORTHWEST — The hunt is on. Garrett McNamara hurtles 60 mph down an icy river in the Northwest, throttling a 255-horsepower Sea Doo watercraft loaded with surfboards. His 11-year-old son Titus clings to his back.

McNamara punches it up to 65 mph to cross the mudflats of a coastal inlet. The engine screams. Eyes tear up in the cold. The water is just inches deep.

McNamara, 41, doesn't worry about hitting a rock or submerged log. He is not a man given to thoughts of mortality.

The O'ahu resident has surfed some of the biggest, heaviest, ugliest waves on Earth — notably the debris-strewn tsunamis generated by crashing glaciers in Alaska. He has broken his back and three ribs, popped both his knees, scraped much of the skin off his thigh, suffered countless sprains and deep-tissue cuts, and shattered his foot several times.

On this January morning, he is here to surf "the slab" — not so much a normal ocean wave as a sudden violent tear in the fabric of the ocean.

Veteran surfers grimace at its disfigured shape. When it rolls out of the deep, it does not rise, but sucks up all the water in its path. The result is not a wall with a front and back, but a hole. The great volume of the North Pacific masses up behind the sub-sea ledge — and then slams shut.

McNamara rode this slab once before, a month before. When the ledge came down, he was still in the hole. The wave pile drove him into the rock reef. He flew back home to Hawai'i with 12 stitches near his right eye. The doctor had pulled a sea urchin spine from the wound.

Such ogreish waves have opened a new frontier in a sport instilled like few others with the mystique of exploration and, increasingly, battle.

SEARCHING THE WORLD

The great hunt for the unknown beast is taking elite surfers to remote coasts around the world — Chile, Iceland, Tasmania, Scotland, Canada, Ireland, Alaska, even Antarctica — and closer to California — rock islands off Baja California, isolated reefs from Point Conception to Eureka, and the Channel Islands.

The quarry is hard to catch, a combination of wind and swell, interacting with the complex bathymetry of the sea floor.

Modern forecasting helps the surfers pinpoint the target. Corporate sponsorship delivers them fully equipped to the destinations. Skis tow them onto giant waves moving too fast to catch by paddling. Life vests and helmets help them survive.

"Surfing always has these periods of dormancy and periods of quantum change," said Steve Pezman, publisher of Surfer's Journal. "Slabs have opened a whole new realm of waves. That's driven a really druglike, addictive exploration."

Famed waterman Laird Hamilton ushered in the era of the slab in 2000 at Teahupoo, Tahiti, when he was towed into a wave with a 10-foot-thick lip of water pitching over him. The headline in Surfer magazine: "oh my god."

Now magazines and Web sites present images that horrify and inspire regular surfers.

In southern Tasmania, locals discovered a yawning, gurgling slab called Shipstern's Bluff. The wave face splits wide open with a sinkhole as it rolls over juts and crags on the sea floor. The lip lunges out as thick and horizontal as a bridge girder. The resulting tube is a cavern, with hard angles, big enough for a truck to drive through.

In the past, the spot would have been unridable on big days. Now, towing gets surfers down the face faster and foot straps keep the boards attached as the riders go airborne over bumps and holes on the surface.

That flirting with catastrophe in extremely shallow water produces "a massive adrenaline rush," said Marti Paradisis, 25, who has ridden it since 2004.

The rush, mixed with the ethereal sensation surfers get inside these giant chambers of water, becomes an addiction.

McNamara said being in a tube is like "time standing still," his mind never more clear and focused.

PRO NICHE

McNamara discovered surfing when he moved to O'ahu from Berkeley, Calif., at age 12. When he graduated from high school, he entered surfing contests.

When he married and had a daughter and son, he started thinking about his future. As with any aging athlete, his prospects were dimming. He didn't have the modeling good looks that guys like Laird Hamilton have. With a hawk nose, freckles and dark hair, he looked more like an Irish soccer hooligan than the golden surfer archetype.

He opened a surf shop on the North Shore and spent long days trying to make it work. He enjoyed signing autographs and holding forth behind the counter in surfer pidgin.

But he couldn't give up the thrill of riding big waves.

In 2002, he decided to give it one more try. He and his tow partner won a contest in waves up to 60 feet high off Maui. They took home $70,000, and McNamara rebranded himself as a big-wave gladiator.

Now he is among a dozen or so surfers who make a living trying to find the outer limit.

"He's never really seen riding normal waves," said Marcus Sanders, editor-in-chief of surfline.com. "He's created a niche for himself as he's gotten older in this big-wave, tow-surfing extreme."

GLACIER WAVES

Top athletes live off the publicity their feats can bring to sponsors. McNamara retains a Santa Monica, Calif., lawyer-manager and has a publicity kit listing media coverage.

But locals and workaday surfers who discover these breaks don't like publicity. While some take pride when a pro shows up, others are furious to find cameras and video spoiling the atmosphere.

And there is a deeper conflict. For many, the purity and romance of surfing comes in pushing their limits to meet an inner challenge, not to score a photo-op. Yet surfers have publicized their exploits for a half-century, selling the mystique to the wider culture, smudging the line between promotion and adventure.

McNamara stumbled onto that line in Alaska in summer 2007 when he attempted to surf the waves created by a calving glacier in the Copper River.

Filmmaker Ryan Casey pitched to him the idea of making a glacier-surfing movie. McNamara jumped at it.

In August, he and tow partner Keali'i Mamala headed out into the silty current as the 300-foot glacier wall shrugged off ice chunks the size of apartment buildings. As they buzzed around, a 100-foot-wide tower of ice broke loose. To their horror, it did not collapse straight down, but fell over like a bookshelf. It hit the river and exploded with the crack-and-boom of thunder. Slabs of ice as big as vans cannonaded the river for hundreds of yards.

The huge displacement of water pushed out a wave like a ripple in a bath tub. Mamala raced to tow McNamara into it. The wave kept rolling without breaking. McNamara dropped the rope, did a few turns, lost his momentum and sank to a stop.

The glacier was too unpredictable. One of the chunks — which they later calculated were traveling up to 200 mph — could have killed him instantly.

"You don't know what's going to fall, how it's going to fall," he said. "You don't know if it's going to fall on you."

He told Casey he wanted to leave. He called his wife and broke into tears.

But sponsors and investors were involved. They had to try again.

"I was no longer in this for the rush," McNamara said. "I was in for the production."

Over the next five days, they raced up and down the river catching sizable breaking waves while trying to avoid the detonation zone. McNamara says he would never do it again.

"I could have died so easily," he said. "My children would not have a father."

THE LIFESTYLE

McNamara knows he can't ride these waves forever and finds increasing pleasure in mentoring the next generation. He's towed Titus into big waves in Hawai'i, and boasts how the boy took a beating "like a man."

McNamara said he doesn't particularly want his son to follow him, but they spend so much time together that his lifestyle might be imprinted in the boy.

"He sees how great my life is," McNamara said.

Titus, who seems more cautious than his father, shrugs.