The grows are almost invisible, hidden deep in national forests. (Growers seem to avoid the country’s national parks, because they are more heavily touristed and monitored.) Cartels set up temporary encampments, leaving a few intrepid workers to oversee tens of thousands of marijuana plants. When the plants mature, teams come in, harvest and dry the cannabis, and pack it out on foot. The sites are difficult to prevent and, due to their isolated and atomized nature, difficult to stamp out, the Forest Service agent Kevin Mayer told me while we trekked in the woods.

The grows are also sophisticated, if not much to look at. Growers identify pockets in the forest that have ample natural light—such as burn scars on southern-facing slopes—as well as water from natural springs, lakes, or streams.

In subtle ways, the trespass grows pervert natural ecosystems: They are greener, wetter, and emptier than the surrounding forest, attracting animals to them. “Our camera data shows that animals use these sites with higher frequency than they use the normal forest,” said Greta Wengert, a founder of the Integral Ecology Research Center (IERC), which has done extensive studies on the environmental damage from trespass grows, as she picked through the detritus the cartels left behind.

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Trespass grows also hurt the natural environment in not-so-subtle ways. Growers divert an estimated 9 billion gallons of water a year, enough to supply 30,000 homes. In some places, that diversion accounts for a quarter or even half of a given watershed’s total surface flow. To protect their plants, the cartels use rodenticides and pesticides that are banned in the United States. These chemicals are eaten by animals, and make their way into the groundwater. More than 90 percent of California’s mountain lions have been exposed to pesticides, as well as 80 percent of Pacific fishers and 70 percent of northern spotted owls.

Gabriel Mourad, the IERC’s other founder, told me that the problem is systemic: dangerous anticoagulant rodenticides are now everywhere in the forest’s food chain. They are in the soil that gets eaten by bugs that get eaten by birds. They are in the small rodents that get eaten by fishers that get eaten by coyotes. They are in the water, hurting fish. And they are hurting people, too.

That likely includes the growers working with the cartels—including, potentially, the kids who seem to come visit their parents on-site, judging by the toys and tiny clothes in the detritus. At the site in Shasta-Trinity, there had been pesticides near where the growers were preparing and eating food, making trace contamination likely. Scientists and law-enforcement agents working on these sites are also being exposed to noxious chemicals. Mayer described fellow officers’ respiratory and eye problems, and said they all worried about the increased risk of cancer.