I put in earplugs, wrapped myself in a blanket, and hoped for the best.

Zack, my guide, fired up the Bombardier ("Bomb" for short), and started humming noisily along the eastern rim of Yellowstone National Park's snow-covered basin. I rode shotgun in this yellow, 10-passenger contraption on tracks, holding my feet close to the front floorboard heaters for warmth. Bombardiers were developed in 1937 by Joseph-Armand Bombardier for mail delivery, freight, school, and ambulance transportation in heavy snow, but went out of production in the 1980s. The surviving models now have a more frivolous, but educational and insightful purpose of winter safaris in North America's most popular national park.

Taking this tour with Xanterra, one of nearly a dozen companies with snow-coach permits, was a once-in-a-lifetime opportunity because there's no place like Yellowstone in the winter. Where else can you see the juxtaposition of a boiling volcano with deep snow? Where else can you cross-country ski around a hot spring or a herd of bison? Just 3 percent of Yellowstone's 3.5 million annual visits occur between December and March, when all roads except in the park's Northern Range are closed to car traffic. That means there's a better chance of getting up close and personal with wildlife and geysers without the crowds or bears.

Over the course of the daylong excursion of the park's lower loop, from the park's south entrance at Flagg Ranch, east around to Old Faithful, I'd come to new, thoughtful conclusions about my excursion. But at the time, all I could think about was that it was 3 degrees outside. As the Bombardier cruised slowly along groomed roads, snow was stacked high enough beside us that numbers on speed-limit signs were not visible. Ice sparkles were like glitter on snow and trees. We passed an avalanche warning sign. Remnants of lodgepole pines, which weathered the 1988 fire that burned a third of the park, stood straight out of the snow like steadfast soldiers, surrounded by thick new trees. Our Bomb and a few passing snowmobiles were the only vehicles on the roads as we headed north into the Lewis Canyon and across the Continental Divide. The sun fought through "geyser fog" that made the sky look hazy.

With molten rock three to five miles underneath (on other continents, it would be about 40 to 50 miles), Yellowstone is considered a supervolcano. If this 34-by-45-mile baby ever blows anything like it did 640,000 years ago, it could cause some serious damage. And yet, millions go to watch Old Faithful blow water and steam up to 150 feet in the air about every 90 minutes.

While it's possible to see wolves, foxes, and coyotes, I was taken by the hundreds of bison we saw, which migrate toward the geyser basins, where the ground is warmer and where melted snow provides easier access to food. They come and go as they please, none too bothered by our snow coach and gawking passengers within a few feet. Why were they so close to the road? Researchers wondered the same thing: Did the bison stick to them because they were groomed, requiring less energy than navigating thick snow? Or did the roads, which were built near rivers, simply interfere with the natural path to their water source? To find out, two years ago, the NPS put a camera on a stretch of road between the Madison and Old Faithful information stations.

Yellowstone is the only place in the Lower 48 where a population of wild bison has existed since prehistoric times, though fewer than 50 native bison remained here by 1902. Weighing as much as 2,000 pounds, they look slow, but can run as fast as 30 m.p.h. and jump five feet. On several occasions during our tour, bison stayed close to the path, where snow was less dense. Occasionally, commercially guided snowmobilers would prompt them to move out of the way faster by stomping on the gas, which irritated me. Snow coaches would pause briefly before taking off while the bison were still close. "There's no one else on the road, people! Slow down," I thought.

On other occasions, bison peacefully searched for food farther away in wide, white plains - brown spots followed by a quarter-mile-long, two-foot deep trench through the snow. During a stop at the Norris information station, about 30 miles from Old Faithful, a ranger said a young bison had gone down in Elk Park, which we'd pass on the way. It would be a feeding frenzy that few witness - the circle of life. A few ravens and a fox were circling by the time we reached it. The bison's pack was about a quarter-mile away. We waited 10 minutes in anticipation before (gasp!) the bison lifted its head. My excited mood turned somber with the surprise of seeing the animal suffering in the winter conditions. My natural human instinct kicked in: Let's do something! Should we look away? Do you think it's in pain? But this wasn't a zoo; this was nature at work. I had to remember why I'd come.

We got back in our semi-warm, protected Bombardier and headed to Old Faithful, where the tour would end. About 12 miles away, our snow coach and several snowmobilers stopped to let a family of bison cross the road. This time, rather than traveling alongside our path, they were going against it. Zack scoffed at the snowmobilers, who didn't let the bison get far enough away before hitting the gas. I looked where the bison were headed. Confirming researchers' theory, they were following the stream. Turns out we were in their way.

I silently thanked them for their hospitality.