National parks in the U.S., already beset by problems ranging from overcrowding to a huge maintenance backlog, now face a new crisis: illegal marijuana farming. “This is massive-scale agriculture that is threatening the very mission of the national parks, which is to preserve the natural environment in perpetuity and provide for safe public recreation,” says Bill Tweed, chief naturalist at Sequoia National Park in California. “[Growers] are killing wildlife, diverting streams, introducing nonnative plants, creating fire and pollution hazards, and bringing the specter of violence.” Tighter border security measures implemented since Sept. 11, 2001, have led to more domestic marijuana production, including large-scale operations in national parks in Arkansas, California, Montana, Texas, and Utah. These growers aren’t hippies; in many cases, they are members of drug cartels who wear masks and tote automatic weapons. Last year officials made their biggest bust yet in a national park, finding 34,000 plants worth an estimated $140 million in Sequoia National Park. But with budgets tight and staff stretched thin, the National Park Service is under-equipped to tackle this problem.