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Jake Thelen had inched his snowboard down a ridge to follow his friend into the powder of the Birthday Chutes in Little Cottonwood Canyon when the snow around him started to move.

Thelen yelled to Sam Kapacinskas as the surface cracked and opened at his feet. He clung to a small shrub on a ridge west of Red Top Mountain as the slope morphed into a river of snow, twice as wide as a 12-lane freeway, rushing into the area Kapacinskas had just entered.

The cliffy slope was scraped bare to the ground, some rock, ice and dead grass the only leftovers of an avalanche that ripped down two chutes and over the ridgeline between them. The crown hung threateningly above, a sheer wall of snow 10 feet tall in some spots. People die in avalanches that are just a few inches deep.

Thelen called 911 to report that Kapacinskas was buried in a slide he could not imagine anyone surviving.

He was wrong.

More than 1,000 feet below, the torrent of ice and snow that had consumed Kapacinskas churned him out again. He picked himself off the ground and began searching desperately for any sign of Thelen in the sprawling debris field.

Utah's snow experts have described the Dec. 19 avalanche as one of the most astonishing they've ever seen — first, because no one was killed or even injured.

But its path and structure also provide a grim example of the scariest kind of avalanche out there: the kind that happens even when conditions seem safe. Known as a deep-slab avalanche, the slide lets a sturdy, thick layer of snow accumulate over a single weak layer, possibly for weeks or months, and waits for someone in the backcountry to happen upon the perfect spot on the slope to collapse it.

"It's probably a one-in-10,000 event that he found that spot to trigger the avalanche," said Utah Avalanche Center forecaster Drew Hardesty. "And what are the odds for both of them to survive?"

'It'll be beautiful' • For Thelen and Kapacinskas, the trap was set in November.

The earliest snows of the season highlight the topography of the Wasatch, melting into the ground south of the ridgelines, but lingering high on the shaded north faces.

There, Hardesty said, it can change from "the snowflakes we all know and love" to something more sinister.

When a thin sheet of snow rests between the warm ground and the cold night air, the water vapor moves quickly through the snowflakes, "turning them into something like rock salt, and it has all the cohesion and strength of a house of cards," Hardesty said.

A layer of those crystals formed over the Birthday Chutes in mid-November, creating in that spot a fragile foundation before a series of storms covered the mountains with a seemingly ideal snowpack that invited recreationists up to play.

On Dec. 19, Thelen and Kapacinskas took their split boards into the backcountry near Snowbird, where both of them work. The sky was blue and the snow fresh. The avalanche rating for the higher elevations was "moderate" — the second-least dangerous on a scale of five.

"Skiing and riding conditions are about as good as they get," the avalanche forecast advised.

Kapacinskas recalled thinking: "Even if we don't ski a lot today, we'll go out, poke around, get above 11,000 feet, take some pictures — it'll be beautiful."

The two men, both 29, covered a lot of ground, hiking over multiple peaks and riding a bit.

They reached the top of Birthday Chutes about 5 p.m., "behind schedule and ... kind of tired," Thelen said. Throughout the day they had been speaking by radio and planning every stage of the journey. But at the chutes, they found themselves running out of daylight. They didn't check their radios, and they didn't discuss their route in detail — a norm in the wintry backcountry, where you never want more than one person at a time in a potential avalanche path.

"We cut corners a little bit," Thelen acknowledged.

As Kapacinskas dropped in the chute, Thelen promised: "I'll follow you."

Kapacinskas thought, "These are some of the best turns of my life. Oh, my God, this is incredible."

He began to turn again when suddenly everything went black. Submerged in snow, he could feel himself moving, but he was too disoriented to try to swim to the top.

"I couldn't breathe," he said. "I was like, 'Jake's got five or 10 minutes to find me, [or] I'm going to suffocate. I'm probably going to die.'

Hidden 'dragons' • At an avalanche-safety workshop in January, Hardesty focuses on the major peril of backcountry decision making: "High-consequence, low-probability events."

"We like things to fit into a nice predictable box, and I'm here to tell you: Snow is unpredictable," he told about 100 skiers and snowboarders gathered in Black Diamond's Holladay showroom.

Not many people die in predictable avalanches; if slides are tumbling down all over the mountains, as they do in heavy storms, staying off avalanche terrain is an easy choice. But the weather improves. The snowpack settles. Danger gradually subsides. Disaster is always possible, but at some point the scales tip in favor of adventure.

Hardesty mentions the Birthday Chutes slide. It draws an immediate reaction.

"Intense," one person says. Another gives a low whistle.

The avalanche has become well known among Wasatch backcountry skiers, both as a reminder of the bargain they make with nature and as a template for discussions about risk. Different skiers have different limits, but facing a possible outcome like the one in Birthday Chutes begs the question: Is skiing that line ever worth it?

The information Thelen and Kapacinskas had at the time doesn't point to an easy answer.

"There's no signs of instability. There's people skiing similar slopes," Hardesty said. "Any snow pit test would have said it was stable snow. There's no collapsing, no cracking."

Deep-slab avalanches have accounted for two-thirds of the avalanche fatalities in Utah since 1940, Hardesty says — mostly because they are so difficult to anticipate once the crumbly, crystalline layer is well-buried. The more snow that covers it, the better insulated it is from impacts above.

"You can have these layers not be reactive to explosives, snowmobiles, or many skiers on the slope at one time," Hardesty said, "and it's only through certain conditions these dragons reawaken through a significant trigger."

Mark Staples, avalanche center director and lead investigator in the Birthday Chutes slide, suspects wind made the snow cover uneven and Kapacinskas turned on an unusually thin spot over the already-stressed bottom layer, causing it to buckle.

"Those faceted crystals collapse, the bonds break, and it's almost like a sheer surface," Staples said. "There's no more friction. There's nothing holding that slab to the mountain."

Unlike avalanches that fall predictably downhill, deep-slab avalanches can propagate in all directions as the cracks spider across the slope — even jumping over a ridge in Birthday Chutes.

Before the avalanche began, Thelen watched Kapacinskas veer left toward the seemingly protected ridge and positioned himself at the top of the chute, 600 feet above.

As he inched forward, the slab pulled away from the top, as if in slow motion.

"I saw the fracture line shoot down ... 18 inches in front of my board," Thelen said. "It was way more massive than you can ever imagine it being."

Thelen leapt toward a shrub poking a foot or two out of the snow and held on while the slab below rocketed 1,100 feet down the mountain, about 80 miles per hour, on top of his friend.

'I'm alive' • Kapacinskas lifted his face off the snow. For the first moment since the avalanche had sucked him into a torrent of frozen debris, he knew which way was up.

He saw Columbine Bowl stretched out in front of him as he waited to be sucked back under the surface. This was his chance to breathe and take one last look, he thought, as a new wave of white rolled over him and swallowed his view.

When the powder cloud dissipated, Columbine Bowl reappeared. Kapacinskas realized the avalanche had stopped, placing him on top. His snowboard was still on his feet. His only losses were his goggles and his torn backpack. Its frame, which followed just inches behind him through the slide, was bent and mangled. But he was not.

"Coming up on top and having that moment to be like, 'I'm — I'm alive! Oh, my God ...' " Kapacinskas recalled. "My next thought was, 'Well, where's Jake?' "

His radio calls to Thelen went unanswered. Kapacinskas couldn't see him on the ridge. He trudged back and forth over the debris field, his legs plunging hip-deep into snow slabs as big as cars and small buildings. He screamed Thelen's name and searched desperately for a signal on his avalanche beacon.

"It's getting close to the time where if I don't find him, he's probably dead," Kapacinskas said. "I was like, 'What am I going to tell his wife?' "

He called 911. Over the next hour of calls with Salt Lake County emergency responders, Kapacinskas kept searching. He also was perplexed as to why they repeatedly called him "Jake."

No, no, he told them. Jake is the victim.

About 1,000 feet above him, Thelen had grabbed his radio but found the battery frozen. He crept down to search for a route to the debris field. He was standing on a crown eight to 10 feet off the ground, its walls looming 730 feet across the chute. He wondered: Was there a way down that would not put him in the path of any more collapsing snow? Was there any hope Kapacinskas still was alive?

Thelen phoned 911. Dispatchers told him not to leave the crown, and help would come soon. He agreed to wait.

Below, in the debris pile, Kapacinskas had been scanning the slope for about an hour, when he received a cautious call from an officer.

"Tell me again what your name is, and tell me again, who got caught?" Kapacinskas remembered the officer asking. A moment later, the officer called back and confirmed: Thelen was alive.

"I remember sitting down on my pack and just being like, 'Whooooooo!' "

Kapacinskas waited happily for his helicopter ride back to the valley.

Thelen, however, remained alone on the ridge. No patrols had arrived. He called back and forth to dispatchers until his phone froze and the sun set. He was certain any chance to rescue Kapacinskas was gone. With no communication and temperatures dropping, he rode west to Tri Chutes and picked his way through the trees in the dark to the road in White Pine Canyon.

Rescue vehicles had swarmed the trailhead when Thelen arrived. He asked an officer if they had found his friend's body.

When the officer replied that Kapacinskas was alive and being picked up, Thelen asked again in disbelief.

"He's in the helicopter," the officer said. "You'll see him in a couple of minutes."

The worst and best fortunes of the Wasatch had funneled together to deposit two friends alive at the trailhead. Thelen doubled over and wept.