The Canadian Coast Guard icebreaker Amundsen weaves in graceful slow motion through the ice pack, advancing through the legendary Northwest Passage well after the Arctic should be iced over and shuttered to ships for the winter.

The fearsome ice is weakened and failing, sapped by climate change. Much of the trip, crossing North America from west to east through the Northwest Passage, will be in open water, with no ice in sight.

The Amundsen is here to challenge the ice that has long guarded the legendary Northwest Passage across the roof of the Earth, and to plumb the scientific mysteries of an Arctic thawing.

A relentless climb of temperature — 5 degrees in 30 years — is shrinking the Arctic ice and reawakening dreams of a 4,000-mile shortcut just shy of the North Pole, passing beside the Arctic's beckoning oil and mineral riches.

More ships will bring the risk — the certainty, some say — of accidents and black oil spills smeared on the white Arctic.

"This water is our hunting ground," Maria Kripanik, an Inuit born 52 years ago in a tent on the beach of Igloolik, told researchers who visited from the ship as it passed her village. There, hunters still use harpoons to snag beluga whales. "I don't know if the people here will like the idea of seeing ships all the time in our hunting ground," she said.

Equally wary are the scientists packed aboard the Amundsen. They occupy the Coast Guard ship for three months each year to study climate change in the fragile North, where the effects of a warmer globe are being felt first. They began this summer in Quebec City and churned west to the Beaufort Sea. As fall came on frigid gusts, the ship turned east again toward the Northwest Passage.

The Arctic ice pack rarely tolerates intruders in late October. It splintered the wooden ships of early explorers who stayed, seized fast the steel vessels that followed, and mocked dreams of regular transit through any of the routes in the maze of straits and channels of the passage.

British explorer John Franklin, whose search for the Northwest Passage transfixed the Western world, perished on a frigid island near here in 1847. Searching for him, many others fell. Their diaries, sometimes found by their frozen bodies, are grim accounts of waiting for a brief break in the ice, as starvation, scurvy and madness claimed them one by one. The map of the Arctic is littered with their names.

The Amundsen is 323 feet long, with engines nearly three times more powerful than normal. The propellers, rudder and hull are hardened. The S-shaped bow rides up on the ice, using the ship's 8,500 tons to crush down through the pack. The Amundsen can maintain a steady march through ice four feet thick and can go through scattered 10-foot floes.

Its nemesis is old ice. Leached of salt, multiyear ice is concrete-hard. Capped by deceptively fluffy coats of snow, its swollen blue belly under the surface can weigh as much as a building.

The captain of the Amundsen, Cmdr. Alain Gariepy, 43, recalls with a shudder a Greek vessel limping into harbor with a 65-foot gash in its hull, torn by old ice.

A half-day east of Kugluktuk, once called Coppermine, the Amundsen meets a flat, gray plate on the water, new ice formed this year. Curious seals poke their heads above water. A white Arctic fox, caught in the ship's spotlight at night on the ice, freezes and then flees. A young polar bear, apparently awakened as it slept on a floe, scampers from the path of the vessel, then ambles on the ice alongside for a while. For one month in September, if the last winter's ice has finally melted and before the new ice forms, ships nose tentatively into parts of the Northwest Passage. Barges bring supplies to Inuit communities and mines. Last year, seven cruise ships poked around the eastern fiords. Icebreakers from Canada, the United States and Russia ply the waters. Only seven ships made it all the way through last year, two of them icebreakers. And none so late as this voyage by the Amundsen.

Abreast of the island where the frozen skeletons of Franklin's ice-stranded crew were found, the Amundsen enters Icebreaker Channel, a slim corridor past the southeastern tip of Victoria Island opens into the path of the vast ice pack flowing south from the Pole. The vessel avoids the largest floes and plows over others with a shudder and a bump.

"A seven-foot-thick ice chunk the size of the ship weighs 4,000 tons," Gariepy explains. "You don't just slam into it; you need more finesse. Even in an icebreaker, if you can avoid the ice, you do."

The ship emerges to head for Bellot Strait, a narrow channel usually choked with ice. Before edging in, Gariepy sends the little red Messerschmitt helicopter from his stern deck to scout.

"This is always the worst place for the ice," says pilot Michel Fiset, 57, as he lifts his aircraft off the ship. He buzzes through the strait, then climbs to view the expanse of gray water beyond. "This is very unusual. We can see 10 to 15 miles and we don't see even an ice cube. It's open."

The Amundsen cautiously approaches Baffin Island at Fury and Hecla Strait, a dangerously narrow half-mile-wide passage. No ship has gone through this late, the captain says. But the Amundsen sails through in clear water. At the eastern mouth of the strait, the residents of Igloolik are surprised the ship is coming through the Northwest Passage in late October. They are not pleased at the weather. They count on a frozen strait to travel to Baffin Island to hunt caribou.

"We get tired of eating seal meat and walrus by this time," Michael Immaroitok, 38, tells visitors from the ship who helicoptered over to Igloolik, a village of about 1,600. Fishing boats are pulled onto the shore; dogs are gnawing on the carcass of a whale.

When hunters bring in whale, or narwhal, villagers share, and the animal ends up boiled, pickled, chopped like salad and served raw — muktuk. But the hunting has been disrupted by "weird, crazy weather in the last five years," Immaroitok complains.

Some believe the worries are overblown. "I think the passage is going to be used, but I don't think it's going to be a navigation highway," says Frederick Lasserre, a professor of geography at Laval University, onboard the ship. Costs of operating in the North are high, the ice cover is never certain and shipping companies do not want to risk delays, he says. "In 20 years, there might be less first-year ice. But there might also be more icebergs breaking off the ice cap that would be navigational hazards."

Michael Byers, an international-law expert at the University of British Columbia who is also on board, is less sanguine.

"The reputable shipping companies would not come here" until the risks of icebergs are low, he acknowledges. "But my worry is the tramp steamer with a single hull under a Liberian flag and Philippine crew. You dangle a 4,000-mile shortcut in front of them — that means time and money. ...

"They run into an uncharted rock, and all of a sudden it's Exxon Valdez times ten," he says.