DEATH VALLEY NATIONAL PARK — Here in ghostly Skidoo, the holes and tunnels are everywhere, nearly a thousand of them puncturing mountains and cratering the desert. Cold winds blow through darkened shafts. Bats flutter in and out at twilight.
Linda Manning, an expert on abandoned mines at Death Valley National Park, peered into a tunnel braced by beams near the old mining camp.
"It kind of gives me the creeps," she said as dank air rushed over her. "These timbers are probably over 100 years old. You never know when they've reached their tipping point."
No other park in the nation has as many abandoned mines as Death Valley. Officials put the number between 10,000 and 50,000, or about a third of all hazardous mines within the national park system.
Now Death Valley and other parks are under increasing pressure to reduce the risks of those mines, risks that include falling, drowning, explosion and asphyxiation. At least 33 people have died between 1999 and 2007 in abandoned-mine accidents on federal and private land, experts say. Many of the openings are all but invisible.
"We are gravely concerned that the Department of the Interior has put the public's health and safety at risk by not addressing hazards posed by abandoned mines on federal lands," said a report by the department's inspector general last July. "We identified serious environmental and safety hazards where members of the public have been killed, injured or exposed to dangerous environmental contaminants."
California parks have the highest number of abandoned mines, with Death Valley followed by Mojave National Preserve and Joshua Tree National Park.
In September, Death Valley closed the defunct Keane Wonder Mine after the report cited a family with a toddler playing near a collapsing opening. In 1984, a visitor fell 30 feet down a shaft in the mine and died.
The park, the largest in the nation, warns visitors about the dangers, but the message isn't always clear.
At the entrance to a mine shaft here large enough to stand in, signs bearing skull and crossbones warn of death from falling, suffocating and explosion, but a trail leads visitors to the opening. Mines, in fact, are often highlighted on park maps.
"I share the concern about mixed messages, of doing tours of mines and then telling people they cannot go into them," said Manning, a wildlife biologist who helps identify and mitigate mines in the park. "The signs say 'stay out,' but you can see the trails go right to it."
Some of the most dangerous mines have been fitted with bat gates that keep people out and let wildlife in. Others have metal netting stretched over openings.
The financially strapped National Park Service estimates the total cost of making mines safe at about $233 million, with an immediate need for $60 million. Sen. Dianne Feinstein (D-Calif.) has introduced legislation to establish a cleanup fund by requiring all hard-rock mining firms operating on public land to pay a 4% royalty on existing operations and 8% on new ones.
The inspector general recommended that parks request enough money to mitigate the worst sites and share resources to shore up their programs for abandoned mines.
Despite warnings, people continue to explore mines and occasionally post their exploits on YouTube. In one video, a middle-aged man heads into a Death Valley mine. At the point when he sees danger signs, a message on the video reads: "Danger. . . . That's my middle name! Let's check out this mine."
"It's not a matter of if we have another death, but when," said Mike Cipra, California Desert Program manager with the National Parks Conservation Assn. "This year, Death Valley got no money for mine reclamation. All national parks are competing for that money. There is an $8-billion maintenance backlog in the national parks."
The job is daunting, especially in Death Valley.
Miners once moved like termites over this harsh landscape, blasting and boring their way through sand and rock. Everything was taken -- uranium, silver, talc, borax.
When the Earth stopped yielding its treasures, the miners simply packed up and went away, leaving gaping holes behind.
Deciding how to balance this rich if treacherous history with public safety is complicated, said Linda Greene, who works for the park's cultural and natural resources department.
"Mining has been going on in Death Valley for more than 100 years," she said. "They are good educational spots, and we consider that when we decide whether to close them or not."
Greene and Manning were searching for old mines in the remnants of Skidoo, once home to 700 people and one of the most productive gold camps in California. The site is one of dozens scattered across an epic, near-biblical landscape.
It didn't take long to find mines. Some were mere holes in the ground, others yawning chasms in the desert crust. Trails led to a largely intact old mine, with signs warning visitors not to enter.