Rangers in Sequoia National Park have swapped their Smokey Bear hats for helmets, weapons and camouflage.
They're preparing to battle the most dangerous threat ever to invade the park: violent Mexican drug cartels growing marijuana at Sequoia and on federal lands across the country.
"What's going on out in the marijuana fields in these canyons is a war of sorts," says Bill Tweed, chief naturalist at Sequoia Park.
As CBS News Correspondent Bill Whitaker reports, it's a war that's growing increasingly dangerous for park rangers and visitors.
"Areas of the park are not safe to visit," says Tweed. "These folks out there are armed every bit as well as we are and in some cases perhaps better."
Once, park visitors only had to worry about bears. Now rangers hunt drug thugs with guns. Last year, Mexican drug smugglers gunned down an Arizona ranger. In California, two backpackers, one at Sequoia, were held at gunpoint.
When beefed up borders made smuggling into the U.S. too hard, Mexican drug lords moved north, planting a marijuana industry on U.S. soil, deep in national parks and forests.
"These groups canvassed parts of the west looking for the perfect place, and unfortunately for us, they decided we offered most of what they needed, and last year they made a big invasion," says Tweed.
The growers plant the marijuana in the most remote parts of the park. CBS News followed some rangers more than two hours almost straight up the forest to hidden ridges and groves of marijuana.
"Nothing but bud, that's what we're looking for," says one ranger.
The plots are well tended. Two suspected growers were arrested this day. They're always well protected and there are always weapons present.
It's more than a cash crop; it's a cash cow. One plant brings $4,000 on the street. Last year, agents pulled $1.5 billion worth from federal land in California alone - a tenfold increase from nine years ago. They expect to exceed that this year.
"You can't get it all, but you go little by little," says Val Jimenez, of the California Bureau of Narcotic Enforcement.
So agents are chopping and hiking into the wilderness, waging an escalating war on a dangerous weed.