Ok folks, sorry it's the Daily Mail but I've never seen or heard of this before
Avalanche control expert Mike Stanford told The Seattle Times: "This is no joke. We did not build it. They are a natural occurrence in nature."
He said he had only seen the phenomena once in his 30-year career. He explained they form when there is a hard layer on the snow, covered by several more inches of dense snow. "Then you add a steep slope and a trigger such as a clump of snow falling out of a tree or off of a rock face," he said.