JACKSON, Wyo. — An unknown number of dud artillery rounds litter the slopes above a remote pass inside Yellowstone National Park, but there are no plans to find and safely detonate them or stop the avalanche control program that uses artillery shells to keep the pass open during the winter.

Yellowstone officials don't know how many of the duds, capable of propelling deadly shrapnel hundreds of yards, lie on the slopes of 10,000-foot Hoyt and Avalanche peaks on the east side of the world's first national park although one National Park Service environmental study puts the number around 300.

At least one artillery shell was found on the highway that goes through Sylvan Pass, an examination of park records obtained by the Jackson Hole News & Guide through the Freedom of Information Act shows.

Despite the danger, Yellowstone's proposed winter use plan would continue the practice of using artillery shells to keep the pass open over the winter to allow no more than an average of 29 snowmobiles and snow coaches a day traveling from Cody.

Officials at the Greater Yellowstone Coalition, an organization that's long grappled with Yellowstone over wintertime use, acknowledge they have not made Sylvan Pass artillery operations a central part of their campaign. Nonetheless, the group has opposed the practice.

"Nothing says 'Yellowstone' like a howitzer and unexploded ordnance," coalition spokesman Jeff Welsch said. "In all seriousness, most Americans surely would be stunned if they knew about this dangerous and costly winter rite in the world's first national park, and that they are paying for it."

But Jack Welch, a leader of the snowmobile advocacy groups called the Yellowstone Task Force and the Blue Ribbon Coalition, said the artillery is necessary to maintain safe access during the winter for people entering the park from the east entrance.

"It's been going on for a long time a long time, as long as I can remember, probably 40 years or so at least, and there have been no issues as far the shells that have not gone off," Welch said. "The few that have been found have been dealt with and haven't presented a problem."

The military-grade explosives, a mix of 105-millimeter howitzer and 75-millimeter recoilless rifle rounds, are mostly unknown to the 400,000 or 500,000 visitors that pass by the area each year.

Areas where duds are likely are closed to the public and marked by signs.

But there has been at least one report over the years of a park visitor finding a dud shell and taking it to a park visitor center. The shell did not detonate.

About 350 inches of snow a year fall in the Sylvan Pass area, and shelling the snowfields of 20 avalanche start zones looming above U.S. 20-14-16 has been used since the early 1970s to trigger avalanches. The idea is to bring the snow down the mountain to protect the hundred or so visitors passing by on snow machines each winter from being swept away by a naturally occurring avalanche.

The Park Service has tried to find and destroy unexploded shells over the years, but the process is costly and the shells difficult to locate in the rugged terrain. The agency also has proposed closing the pass during the winter, but snowmobile enthusiasts and local and state leaders have vehemently opposed the idea.

In addition, the artillery use has been overshadowed by legal battles over the number of oversnow vehicles allowed throughout the entire park and their impacts on air quality, the soundscape and wildlife.

The National Park Service is taking public comment on Yellowstone's latest winter-use plan, touted as a long-term solution and generally accepted by conservation groups. No significant changes are proposed to operations at Sylvan Pass under the plan.

A total of 176 snowmobiles and no snowcoaches traveled over Sylvan Pass on their way to the Yellowstone Interior this past winter. It costs about $125,000 to keep Sylvan Pass open every year, according to Park Service estimates.

The latest document that proposes winter-use regulations in Yellowstone, a 674-page final supplemental environmental impact statement, addresses the issue of "unexploded ordnance" in a single paragraph and estimates the unfound, unexploded ordnance at 300. The document is open to public comment through Monday.

But Yellowstone officials dispute the estimate.

"What the actual number is, that's anyone's guess," Yellowstone Deputy Chief Ranger Nick Herring tells the News & Guide (http://bit.ly/ZJIO2B).

Shoddy record keeping in earlier years prevent an accurate accounting into the number of duds and the number of unexploded shells located and destroyed.

There has been better record keeping in recent years, but just over 20 percent of the faulty howitzer shells launched in the past 16 years by the Park Service have been recovered and detonated, according to information provided by park spokesman Al Nash.

But Nash said even with those numbers it's not possible there are 300 unexploded shells.