Even expert skiers and snowboarders fear an invisible menace on ungroomed hills — camouflaged death traps known as "tree wells."

I saw the bottom of his snowboard next to a tree, and just him wiggling. —Peter Smart, owner and operator of Extremely Canadian in Whistler, B.C.

Enthusiasts familiar with all kinds of terrain were reminded of the perils of tree wells this week when Chris Johnston posted a helmet-cam GoPro video showing him traversing through evergreens on his way down a backcountry run, only to take a spill into a depression of deep, loose snow near the base of a tree.

When the White Rock, B.C., man woke in the darkness of the three-metre-deep cavity, he felt something blocking his lungs.

“I was breathing in snow,” he recalled. Johnson was lucky to survive, but it took him 15 minutes to wriggle free.

Tree wells form during or just after big storms or snow cycles. The general principle, according to the ski-industry website DeepSnowSafety.org, is that the fresher the snowfall, the higher the risk of falling into the hollow spaces created near ungroomed and treed areas.

According to the Canadian Ski Patrol, 90 per cent of people trapped in the hazards are unable to rescue themselves, although many more cases may go unreported.

According to the Canadian Ski Patrol, 90 per cent of people trapped in the hazards are unable to rescue themselves, although many more cases may go unreported. (Duk Han Lee/CBC)

DeepSnowSafety.org estimates that one-fifth of ski-related fatalities are caused by accidents in tree wells and snow immersion suffocation (SIS), in which a victim is smothered by powder snow.

Canada has had two fatalities so far this season from SIS and tree wells, Paul Baugher of the U.S. Northwest Avalanche Institute, told CBC News.

"One was in-bounds at Whistler, another was out of bounds at Pemberton," said Baugher, who works with the U.S. National Skiers Association to publish safety information on DeepSnowSafety.org.

'Umbrella effect'

Baugher said his U.S. data showed there have been three similar deaths in U.S. since January, and that the U.S. averages roughly four SIS-related deaths a year.

What makes tree wells so tricky to spot is the layer of dry, fluffy powder snow that accumulates around the trunks, further obscured by the surrounding snowpack.

A skier demonstrates the depth of a tree well for the U.S. public safety information website DeepSnowSafety.org. (Courtesy DeepSnowSafety.org)

Overhanging branches create an “umbrella effect,” explained Peter Smart, who runs guided ski expeditions for Extremely Canadian in Whistler, B.C.

“Hardwood trees in the East don’t form big tree wells. They’re the result of the umbrellas of coniferous trees,” Smart said. “Snow falls down and slides down, so nothing falls underneath. Once the snowpack is deep enough, it starts to climb up around the branches of the trees, so there’s nothing to see — just tree branches going down into the snow.”

Smart said Extremely Canadian ensures that tree-well safety is part of everyday instruction for his groups. Even so, there’s not much someone can do if they fall in and need rescue.

"Hope you’re not alone. It’s that simple," Smart said, noting that he spotted a snowboarder earlier this week who had apparently fallen head first into one of the tree wells.

"I saw the bottom of his snowboard next to a tree and just him wiggling," he said. "His buddies were there; they got him out."

'Easy to choke on powder'

Iain MacMillan, editor of Ski Canada magazine, remembers tumbling into a tree well decades ago when he went slightly off the groomed run in Whistler to experience fresher powder.

Skiers who explore backcountry runs are advised to buddy up and keep each other in sight in case someone falls into a tree well. (Jonathan Hayward/Canadian Press)

"I'm struggling, I'm choking on snow," he recalled. "It was annoying because more branches and snow kept falling in the more I wiggled."

MacMillan said he managed to get at least one of his skis off and dig himself out after roughly 10 minutes. A skiing companion up ahead was also buried in a tree well, but she also made it out.

"In a worst-case scenario, you’ve got your head down, you can’t get your skis off, and your hands are stuck," MacMillan said. "It's easy to choke on powder snow because it’s so light, dry and fluffy, so you inhale it quickly."

I’m struggling, I’m choking on snow. It was annoying because more branches and snow kept falling in the more I wiggled. —Iain MacMillan, editor of Ski Canada magazine

He advised skiers who explore off trail to unstrap their poles, which can restrict arm movement if you become immersed in snow.

He now skis with a backpack with a whistle attached to the chest strap and also has an Avalung device — an apparatus that draws air from snow so a skier or rider can breathe through a tube in case of an avalanche burial. Radios and cellphones can also be used to call for help.

Tree wells appear to be a problem mainly in Western slopes, according to Ron Cameron, VP of operations for the Ontario division for the Canadian Ski Patrol. He has never encountered a case in Ontario, though he said some incidents may go unreported.

"Is it a big risk here in Ontario? I do not believe so," he said.

Despite the known risks of tree wells, MacMillan said it’s unlikely skiers and snowboarders will keep off fresh powder.

"Just looking at a photo of untracked snow, it’s like it’s calling you," he said. "You don’t hear the metal edges of your skis on the slope because it’s all fresh and light and fluffy. There’s just a primal rush that you don’t get on a groomed run. That's why we go there."