"Treewells" and "snow immersion suffocation"

YouTube link.

The video above documents the rescue of a skier from a treewell.  The following points are worth noting:

Point of clarification - This is real world stuff, raw and flawed. Everyone in this video is a recreational skier, an amateur in back country rescue. Even with a successful outcome, this rescue has mistakes and is not a representation of proper technique. The intent of this video is to demonstrate and educate people on the dangers of tree wells. NOT a demonstration on HOW to rescue someone from a tree well...

With a 15-20 foot snow pack in winter, a lesser known but of equal or greater danger [compared to an avalanche] are "tree wells" - a quicksand like funnel that forms from the far edge of tree branches, tapering down to the base of the trunk. Because the snow around the edge easily collapses, often one falls in... upside down. Once in, all sound is absorbed by the soft snow, screaming for help is useless and any movement draws you deeper down. Also, you are nearly invisible from searchers on the surface - they can't see or hear you and need to rely on the transceivers to locate you - all taking precious time.

The website Deep Snow Safety has an extensive page on the natural history of treewells.

A tree well is a void or depression that forms around the base of a tree can and contain a mix of low hanging branches, loose snow and air.  Evergreen trees in particular (fir, hemlock, etc) can have large, deep tree wells that form when low hanging branches block snow from filling in and consolidating around the base of the tree.  These voids can be hidden from view by the tree’s low hanging branches.

There is no easy way to identify if a particular tree has a dangerous tree well by sight therefore, treat all tree wells as dangerous. 

If you are ever planning to ski deep snow off groomed trails, you should read this page.   Many photos and practical tips at the link.

Addendum:  Video of a father rescuing his son from a treewell.  The kid was luck he landed upright and able to reach up an arm for the father to grab.