On the shore of Lake Tiktalik in Canada’s Western Arctic, the thawing permafrost had set off two huge landslides into the water, leaving yawning craters on the tundra. These “thaw slumps” measured several hundred feet wide and just as deep.

Jaden Cockney, 17, clambered down the side of one slump as his boss, William Dillon, looked on cautiously. Jaden was part of the team that Mr. Dillon, 69, had created to measure the retreating permafrost. Only a few decades earlier, the permafrost had lain just several inches below much of the region’s surface. But now it was thawing so rapidly that it was being pushed further and further underground. Along shorelines, it collapsed into lakes or the Arctic Ocean.

For centuries, the Western Arctic has been home to Mr. Dillon and his ancestors, the Inuvialuit, as the region’s Inuit are called. But these days, the thaw slumps — like the one Mr. Dillon’s team was documenting 10 miles south of their hamlet, Tuktoyaktuk — are the most dramatic evidence of a phenomenon that could turn the local Inuvialuit into Canada’s first climate refugees.

Tuktoyaktuk itself now stands face to face with the Arctic Ocean’s increasingly angry Beaufort Sea, and rests atop 1,300 feet to 1,600 feet of thawing permafrost threatening to sink it.