I wonder how many times it is due to the skier or snowboarder passing over an the unseen roof (like a snow bridge) that collapses into a unseen well ...
You would think the vast majority are like that, because otherwise it would be people just careening out of control into obvious holes.
Why would you think there aren't enough "careening out of control" to generate the toll we've seen in tree wells? The only deaths I've seen reported due to unseen (non-tree, non-stream) wells were in the hollow over the steam vent on Mammoth that formed during a very high accumulation rate storm. Those weren't even Non-Avalanche Related Snow Immersion Deaths (NARSID), but poison gas deaths.
Dale B. Dalrymple
http://dbdimages.com
