Every day this winter, a special team of rangers in Yellowstone National Park begins work well before dawn. Some focus on gathering weather data to get an idea of what they'll face. Then two rangers  suit up in long underwear, heavy insulated pants, jackets and boots, neck gaiters and helmets. At first light, they set out on two snowmobiles, using the buddy system in case of trouble.

They coax their machines up a curvy, snow-covered mountain road -- sometimes through drifts and blizzards, with wind-chill factors of 30 below zero or worse -- to reach Sylvan Pass, an 8,530-foot-high gap between peaks that rise above 10,000 feet.

There, they assess the avalanche risk. Annual snowfall averages nearly 30 feet and the key stretch of road crosses at least 18 avalanche chutes. The rangers inspect snow conditions, dig pits for cross-sections and take other measurements. If they decide there's a substantial risk, they close the pass to snowmobile and snowcoach traffic. Then they bring out the explosives.

Mostly, they use a 105-mm howitzer positioned against a cliff -- Korean War artillery designed to function in the cold. Team leader Maura Longden says, "It's a very good weapon for our conditions."

Longden has to call out reinforcements to operate the gun; at least five rangers are needed. The designated "gunner" yells commands to the rest as they handle shells as big as three Aladdin thermoses. Each sequence ends with: "Ready to fire! … Fire!"

Sometimes, they drop explosives from a helicopter. By either method, they deliver the blasts precisely to make the avalanches thunder down. They use more machinery to clear the debris and groom the snow on the road. Then they reopen the pass to traffic.

The rangers speak of their "mission" as if they're in combat. Every step has a degree of danger. They work long hours in wild conditions and must traverse at least eight avalanche chutes just to get to the gun.

Avalanches have killed six people in Yellowstone over the years. None of those deaths occurred on Sylvan Pass, but rangers have been involved in near-misses there since they began bombing it in 1973. And one ranger died while checking conditions on the pass during a 1994 blizzard. He drove off the road, rolled down an embankment, got trapped under the machine and the snow, and suffocated.

The rangers perform this mission primarily in the name of winter tourism. The road through Sylvan Pass connects to the park's east gate in Wyoming. A trickle of tourists on snow machines flows both directions, from the bulk of the park to the east gate, and from the east gate over the pass to the bulk of the park. It's the only place in any national park where such aggressive avalanche control is used to clear the way for tourists on snow machines.

Many park staffers and avalanche experts think the mission is too risky, too costly -- at several hundred thousand dollars per winter -- and not in keeping with the National Park Service's conservation goals. If they had their way, the pass would be closed in winter. One expert says privately, "I don't think there's a Park Service person who thinks that flailing away at a mountainside with an artillery piece is what they were hired to do."