It was a perfect day to jump mountains. The sky was clear, the temperature mild and the jagged peaks in Canada’s backcountry were dusted with snow.

On that day, April 22, a team of expert snowboarders stood on a ridge. The plan was to pop off the top of the mountain, drop onto its powdery face and shred its untouched slope, racing against gravity for the sheer love of it.

But each of the four riders on the ridge in the Whistler region of Canada that day was also weighing the sobering reality that the terrain at their feet might be engraved with features that would make it susceptible to their worst fear: an avalanche.

And that is what happened.

One of the four riders, Brock Crouch, 18, went on the first run down the mountain, then returned to try a second route nearby. But somehow, he ended up on a cornice — a bulge of snow sculpted by wind that bends over a ridge to form a ledge.