PACIFIC NORTHWEST — The hunt is on. Garrett McNamara of O'ahu's North Shore hurtles 60 mph down an icy river in the Great 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, does not worry about hitting a rock or submerged log. He is not a man given to thoughts of mortality.

McNamara 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 sudd en violent tear in the fabric of the ocean.

Veteran surfers grimace at the sight of 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 Hawaii with 12 stiches 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.

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 Californ ia -- at 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 that is never the same.

Modern forecasting enables the athletes to 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 August 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."

Since then, magazines and Web sites have presented 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.

"I couldn't believe something existed like this in Tasmania -- just the sheer amount of water in the lip," said Marti Paradisis, 25, who has ridden it since 2004.

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," Paradisis said.

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

The late nature writer T.H. Watkins hit on the allure when he described bodysurfing near Dana Point, Calif., as a boy in the late 1940s: "The lip of the wave curved over my head, and for one brief instant I found myself in a long, green translucent tunnel that stretched 40 or 50 feet on either side of me. That moment was when I heard the Sound, a high, hollow, almost metallic keening that cut through the outside roar of the surf until it was all that could be heard. It seemed to come from a great distance, like a cry out of the ancestral night, then swept over me and moved on just as the wave seized my helpless body and plunged it through the water and into the sand. ... When I finally surfaced I was certain I had been privileged to experience one of the essential mysteries. ... I remain as convinced today as I was then that I had heard nothing less than the voice of the sea itsel f."

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

He first discovered surfing when he moved to O'ahu from Berkeley, Calif., at age 12. When he graduated from high school, he entered surfing contests. He was a solid journeyman but never a marquee name.

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 had. 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. A gregarious sort, 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 the island of Maui. They took home $70,000, and McNamara re-branded 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."

That niche is not without tension.

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.

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

In August, he and his tow partner, Kealii Mamala, headed out into the silty current as the 300-foot glacier wall shrugged off ice chunks the size of apartment buildings. The river flowed south about 10 knots, carrying driftwood and ice. 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 t he 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.

"It was the most flourishing, tingling endorphin rush I ever had," he said.

When he came to shore, the electricity in his nerves gave way to a deeper angst. The glacier was too unpredictable. One of the chunks -- which they later calculated were traveling up to 200 mph -- could kill 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 this was not just a surf trip. 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.

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

This winter, he has closely monitored the coast between San Francisco and Southern Alaska, checking buoy readings and weather reports on his computer at home in Pupukea.

In January, when conditions looked good, he and Titus took the red-eye to San Francisco. Three younger surfers and a videographer from the East Coast picked them up with two skis in tow, and two friends from Malibu, Calif., followed with two more.

They made a beeline north into the redwoods. McNamara checked out an offshore rock that had never been ridden. Its location and underwater topography suggested it might see some of the biggest surf in the world.

At a dusty crossroads, his local guides to the remote spot pulled up in a pickup -- only to announce the fog had rolled in. The beast would escape.

McNamara turned his sights on the injurious slab he had ridden a month before. The crew drove up the coast until they arrived here, at this spot in the "Pacific Northwest." Locals insisted its location not be identified even by state.

The swell has not lived up to anticipation. Still, McNamara races down the river as if the swell-of-the-century were rolling in. He wears a San Diego Padres cap backward. Titus clings to his back. His posse follows on three other skis, hollering and laughing.

McNamara has more energy than his young acolytes. He said he doesn't worry about dying -- and leaving his children and wife of 15 years behind. In the ocean, he has a keen grasp of the forces at play and a sixth sense of impending danger.

But he knows he can't ride these waves forever and finds increasing pleasure in mentoring the next generation. He's towed Titu s into big waves in Hawaii, 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.

They punch through the incoming waves at the river's mouth and turn south. The coast is eroded basalt, covered with pine and fern. The water is in the high 40s, and the winter sun is out in rare brilliance.

The slab sits just off a cove roiling with whitewater. Lines of foam drift out into dark green sea.

Rob Brown, 24, of North Carolina, is the first to take the rope. McNamara tows him out where the swells are just faint undulations on the surface -- the energy hidden below. He reads the moving water and accelerates onto a barely perceptible bump.

Within seconds, the gentle wave opens up into an 8-foot ditch. Water rushes off the reef into its maw. The rock bottom becomes ominously visible. McNamara turns hard to sea and whips Brown into the hole.

The surfer rockets off the ledge. He hits the sharp angle at the bottom. The edge of his board snags. He stumbles, over-corrects and gets pulled too high up the face. The thick lip throws over in a cathedral of green light. And he goes up and over the steeple.

"Did he make it?" McNamara yells.

"No! Down, down!" shout the onlookers floating in the channel.

Brown's head pops up in roiling foam. McNamara jets into the maelstrom, drags him onto the rescue sled, and shoots out to catch the next.

Will Skudin is up. He complains that the zipper on his wetsuit won't close.

"Come on, be a man!" McNamara goads.

Skudin, 23, of Long Island, grabs the rope, drops into the hole, gets trounced, and comes up in the foam.

"How was it?" McNamara shouts.

"Mental!" Skudin shouts back.

Over the next two hours, the three surfers find speedy spots and little tubes to shoot through. But every time they streak deep into the cavern, they get swallowed.

The wave today is serious, but not fearsome enough to be a McNamara special. Since no one else has the skill to tow a surfer into a tricky wave, he stays on the ski.

The tide is dropping, and parts of the reef are surfacing. After a few more dumps, the tow-board is battered by the rocks.

"Let's get out of here," McNamara says, "before someone gets hurt."