Deep snow naturally forms "wells" around trees where the snow is much less dense and deep. Unfortunate riders (of skis, snowboards or even snowmobiles) can plunge into these holes and can find escape impossible as surrounding snow collapses on top of them.
From a report on the Whitefish incidents in The Missoulian :-
The promise of fresh snow and a day filled with exhilarating outdoor recreation turned deadly for three men in three separate incidents on Saturday in the Flathead Valley.
Two of the deaths occurred on the slopes of Whitefish Mountain Resort, and the third death happened in a snowmobile-triggered avalanche in the South Fork of the Flathead River west of Hungry Horse Reservoir.
If you ski amongst the trees, be aware of tree wells and - as with all off-piste skiing - never go alone.
"This series of events is unprecedented," said Donnie Clapp, spokesman for the resort "Less than two weeks ago we lost a skier (a 16-year-old German exchange student) who died in a tree well, and before that it had been more than 10 years since somebody lost a life in a tree well up here.
"In addition to this being very unusual, it's been very hard on all of us who work here. Any time we lose a member of the skiing family, it's extremely hard - and to do that twice in one day is, well, there are no words to describe that."
Take care people!