My sisters ex-boyfriends sister (got that?) died in a tree well @ Mammoth a couple years ago. I do not know particulars as I didn't speak to him directly about it at the time but she and her husband were riding together going down a run that they had both ridden countless times (Mammoth was home MTN) through a glade and when he got to the bottom she never followed. He spent hours waiting/looking for her after notifying ski patrol but they still did not find her later that day, even knowing the run she disappeared on. They were both advanced riders, in bounds, at their home resort. It was later found that she actually hit a buried branch with the nose of her board that made her "go over the handlebars" head first into a deep tree well. She completely disappeared and suffocated.
Super fkin scary and utterly tragic. She left two young kids behind. RIP
EDIT: found an article about it http://www.ocregister.com/news/snow-...-ski-deep.html