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NORDEN, CALIF. (Fortune Small Business) -- I'm lying in a hole in the darkness, ice-cold and soaking wet. There's barely room for me to turn from my right side to my left, but with a grunt I manage to twist onto one shoulder and angle the blade of a small metal shovel against the wall of snow a few inches from my head. As I chip at it, icy spray hits me in the face.

The snow is spring slush, heavy and wet, and I'm drenched from a combination of sweat and melting snow. My arms are aching. My back is throbbing. I hack away. After a few minutes, a pile of loose snow has formed around me. I shove it toward my hips with a sodden mitten and then wriggle to push it backward with my knees. I've been at this for hours, and my hole is still scarcely big enough to hold me.

Exhausted, I fall on my back and stare at the white roof a foot above me. Then I rally and squirm back into position. I don't have time to dillydally. Once night falls, the temperature outside will plummet into the low teens. As cold as this cave will be, the outside air is going to be much worse. It may feel like a tomb, but this little snow cave could save my life. If I can finish building it.

I've come here to the Sierra Nevada to take an unusual winter-survival course that is run by Adventure Out, a small guiding outfit based in Santa Cruz, Calif. The weekend program aims to introduce customers to something very simple yet very difficult: the challenge of staying alive through a winter night with whatever shelter you can make on the spot.

Having spent much of my childhood on the rugged coast of Maine, I like to think I can handle myself in the wilderness. As an adult, I've happily endured desert-survival training and backcountry camping trips among the grizzlies of Yellowstone. But the outdoors in winter has always intimidated me.

Maybe my unease stems from reading, at an early age, "To Build a Fire," Jack London's classic tale of a Klondike greenhorn who blunders into the wilderness and freezes to death. Whatever the reason, I'm aware that winter can easily kill the ill-prepared - an adjective that, I hope, will not apply to me come Sunday.

That's the plan, anyway. But between now and then, I've got the little matter of this snow cave to figure out. Not just digging it, but actually spending an entire night inside it.

The fact is, I'm not sure how I'm going to fare in a bedroom where icy walls keep the air temperature several degrees below freezing.

Hours earlier, our group met in the dining room of the Clair Tappaan Lodge, a massive wooden structure that stands on the brow of a mountain 15 miles west of Lake Tahoe. Built by Sierra Club volunteers in the 1930s, the facility today operates as a hostel, with guests helping out with chores and eating communally at long tables. Later, after a hearty dinner of fried trout and boiled potatoes, Adventure Out president Cliff Hodges, 28, commandeered the dining room and our orientation began.

It's the first time Adventure Out has offered this course, so the roster is small. There's a thirtysomething paramedic from Reno plus his mother; a young information technology executive from the Bay Area traveling with his girlfriend; and another IT professional accompanied by his Alaskan husky. As we go around the room introducing ourselves, I'm impressed by the group's extensive experience. The mother-and-son team, it turns out, spent years homesteading in the Alaskan backcountry. We share a desire to understand and experience the natural world in a new way.

We're here to learn how to survive the first 36 hours in the wilderness in winter. In a real-life survival situation, this would be the most crucial interval, the time during which we would likely either be rescued or die. The immediate priorities are shelter and fire.

"I've never heard of wilderness fatalities in the winter that were caused by anything other than exposure," Hodges says. He reviews the basics of how the body shuts down as it's exposed to cold, and he dispels some common myths, such as the idea that you can rehydrate yourself by eating snow or drinking your own urine. "That's never cool," he says, explaining that the toxins consumed would likely cause vomiting and further dehydration.

Hodges explains that snow is a two-edged sword. In direct contact, it can quickly drain your body's heat. And too much snow can prevent you from moving to safety. That's what happened to the Donner Party, trapped by an unseasonably early blizzard at a spot 10 miles from here in 1846. By winter's end, nearly half the party had died, and many others had resorted to cannibalism.

That's the bad news. On the bright side, snow is wonderfully suited for making shelter. "It's a great building material," Hodges says. "It insulates well, and it's very malleable. You can harden it and shape it and cut blocks out of it. You can do all kinds of stuff with it."

In theory, the kind of snow shelter we're going to build is about as simple as it gets. It's called a quinzhee, a word of Athabascan origin applied to a pile of snow that you tunnel into and then clear out from the inside to create a dome, with a bench off to one side that you sleep on. What could go wrong? Plenty. Ideally, the roof of the shelter will be a shell of snow about 18 inches thick. If you pierce the roof, you really can't patch it. Once the integrity is gone, the roof will fall in on you. Hours of labor - pfft!

Back at work after lunch, I continue broadening my dome. As I work upward, the light coming through the snow gets brighter, changing hue from pale white to a bluish wash. Hodges has stuck 18-inch-long sticks into the sides and top of my snow pile, so that if I hit one I know not to dig any further. But I'm still nervous. Under the heat of the sun, the snow is turning to mashed potatoes. Will my roof start to cave in? I try not to think about it. I start building the shelf where I'll lay my sleeping bag over a waterproof tarp and a foam pad. Soon the bench is big enough to test, and I climb up and lie on my back. Being motionless feels delicious.

Hodges has spent the afternoon digging an elaborate fire pit in the snow, complete with snow benches for the entire group and a protective wall to shield against the wind. Gathering round, we stare into the flames as he explains his passion for making stone arrowheads, knives and other primitive implements. Hodges hunts and eats animals that he brings down with homemade bows and arrows. Four months ago, he says, he killed a 450-pound black bear by shooting it with a single arrow.

"I use everything - I tan the hide, I eat the meat, I use the bones to make tools," he says. "I'm still processing the experience."

Eventually the last of the firewood burns down to embers, and we shuffle off to our holes in the snow. As I shimmy through the narrow entrance, the darkness and the cold feel wild and uncivilized, unbound by the normal rituals of human life. For an instant I imagine that a bear crawled in there while I was gone. Once inside, I turn on my flashlight: No, it's just me.

Carefully taking off my snow boots, I climb onto the bench and work my way into the sleeping bag, removing first my snow pants and then my parka. Lying comfortably on my pad on top of the plastic sheeting, I take a deep breath and exhale. Well, here I am at last, I think.

Then I start to slide.

I move about four inches before I come to a stop. My feet are hanging off the edge of the bench. I realize what has happened: Inadvertently I've built the bench with a slight incline. Now that the night air has turned it to rock-solid ice, it's as slick as greased Teflon. Maybe if I lie perfectly still, I can... I slide another inch.

I try to shift back into position with a series of small hops. I move back up, but the pad and tarp only slide further off. This is not working.

There's nothing to do but put my boots back on, take my shovel and start hacking at the errant bench, carving it so that it slopes the other way, back against the wall. Snow and chips of ice go flying as I madly attack the problem. As I'm working on the wall behind the bench, the shovel strikes a stick. Great! All I need now is to finally collapse the dome. But the structure holds, and I'm able to climb back into my bed. I'm surprisingly comfortable - that is to say, less cold and miserable than I expected. For the first time, I realize I actually have a pretty good shot at making it through the night. My room has zero amenities except one: If I need to relieve myself in the middle of the night, I can simply roll over and direct an arc to the floor. The warm urine will dig a hole for itself and then odorlessly freeze.

I drift off to sleep and wake up to find that the entrance of the cave is bathed in pale light. Dawn, already. Yippee!

I look at my watch. It's 1:30 a.m.; moonrise. I've got another five hours to go.

As I lie in the freezing gloom, a little voice becomes increasingly audible, the one that's been murmuring all along: Why are we doing this? I drift off to sleep again. A little later I wake up and realize that the outside of my sleeping bag is soaked from condensation on the tarp. I'm still fairly warm inside the bag, but for how much longer? I concentrate on drifting back to sleep - it's the only way to make the time go away. And then: "Jeff!"

It's Hodges, making the rounds. It's 6:30 a.m. Groggy, I put my boots on and wriggle out into the cold, clear light of a perfect dawn.