They lurk off-trail, sometimes just below the surface — unexploded ordnance fired at the north face of Green Mountain a century ago by Colorado Army National Guard troops.

Now, after a decade of probing 466 acres of Lakewood’s William F. Hayden Park for old munitions, officials have devised a cleanup plan that would use a type of high-end metal detector — known as a “Man-Portable Vector” system — to unearth and remove potentially explosive ammunition from the park, which sees approximately 250,000 visitors per year.

“They are going to cover every square inch that’s accessible,” said Jeff Swanson, the project manager at Green Mountain for the Colorado Department of Public Health and Environment.

The preferred cleanup plan, at $8.6 million, is the most expensive and comprehensive of the six devised by the Army National Guard and CDPHE to address the years-long problem. That’s intentional, says Swanson, especially after several unexploded munitions were found in a concentrated area on the north side of Green Mountain several years ago.

“When we find that many in the study, it tells me we have a concern there are more out there,” Swanson said. “This plan relies more on removing the hazard than trying to manage the hazard.”

In total, 13 “munitions and explosives of concern” — to use Army parlance — have been found and disposed of on Green Mountain in the last decade. There have not been any reports of injury or death from someone coming into contact with any of these objects, but Drew Sprafke, parks supervisor for Lakewood, said that doesn’t diminish the risk.

“The risk is people finding things and picking them up,” he said.

A final decision on the cleanup plan, which will be paid for by the U.S. Department of Defense, won’t be made until early next year. The public is invited to comment on the plan through Nov. 30. Information on how to do so is available here.

Green Mountain was the target for artillerymen with the Colorado Army National Guard training at nearby Camp George West military base. The Guard leased Green Mountain from the Hayden family and used it as a firing range from approximately 1912 through the 1930s.

The men fired 75mm ammunition — projectiles typically filled with hundreds of lead balls that would eject shrapnel upon detonation. The troops also fired high-explosive 75mm rounds, according to the Army National Guard.

Joe Rhodes, an artilleryman for 34 years with the Colorado Army National Guard and now an environmental restoration specialist with the agency, said most of the projectiles back then contained gunpowder as their explosive force.

Green Mountain is not a minefield, he said. The vast majority of what’s been found on the mountain has been munitions debris, as occurred nearly 10 years ago when a hiker came across shell casings.

“If it doesn’t have a fuse on it and you don’t light it, it’s not going to explode,” Rhodes said.

But if it is still intact, he said, the potential is there for trouble.

“The risk is low, but Congress mandated we look at these areas and make it safe for the public,” Rhodes said.

It will probably be a couple of years before crews actually head out to the site and sweep it with their detectors. First the plan must be formally adopted by the Army National Guard, sometime next year, and then money will need to be found, Swanson said.

Clearing the area to a higher degree of certainty, Swanson said, will not only protect visitors but will also reduce uncertainty for fire crews battling grassland fires on Green Mountain, which has seen several blazes in the last few years.

“You don’t want to fight fires on munitions sites,” he said.

Art Morris, whose house backs to William F. Hayden Park in Lakewood’s Green Mountainside neighborhood, said he used to regularly run to the top of Green Mountain until his knees started acting up. He’s not certain an $8 million-plus cleanup is necessary when there has been no record of people being hurt there.

Across the street, Jim Efaw said his yard was inspected a few years ago for any shrapnel or unexploded ordnance from the Camp George West firing exercises. He said he and his sons even found some spent shells when they were hiking on Green Mountain once.

“If they’re going to clean it up, that’s a good thing,” Efaw said. “But I think the neighborhood in general is not concerned about it.”