The Napa Valley of pot faces an unlikely challenge: legalization.

The drive to Casey and Amber O’Neill’s HappyDay Farms winds up a dirt track off Highway 101, three hours north of San Francisco. The road climbs to 3,000 feet along a ridge with stunning views of pine-covered mountains and the blue band of the Pacific Ocean, 25 miles to the west.

As I turn down the O’Neill’s pitched driveway, a barking Great Dane–Catahoula named Emma rushes me, hackles raised. Casey and Amber insist she’s friendly, but it be would difficult to approach the O’Neill homestead undetected. And that’s by design.

The O’Neills grow cannabis in northwestern California’s infamous Emerald Triangle, a densely forested region of labyrinthine back roads, secret valleys, and perennial creeks. For more than 40 years, it’s been a great place to hide out and grow a prohibited but highly desired product — not just for the O’Neills, but for scores of other off-the-books growers, many of whom have been farming here for generations.

Casey O’Neill was born 35 years ago on this hilly, 20-acre spread. One acre of his property is flat enough to produce vegetables and strawberries, which he sells to nearby restaurants and members of a CSA. But it’s the rows of cannabis that bring in most of his income. His brother and father grow the same lucrative product on adjacent properties: a well-known hybrid strain called The Great Success, which took 11th place out of more than 650 entries at last year’s Emerald Cup, the state’s premier pot competition.

The Emerald Triangle is the Napa Valley of cannabis. Blanketing more than 10,000 square miles in Trinity, Humboldt, and Mendocino Counties, the region produces about 60 percent of the nation’s pot, most of which heads out of state on the black market.

And just as winegrowing and tourism dominate Napa County’s economy, so does cannabis dominate here, helping to fill the void created by the collapse of the once-robust fishing and timber industries. In Humboldt County, the region’s heart, researchers estimate that cannabis provides a third of private-industry revenue; in the triangle at large, the California Growers Association, a cannabis trade group, says every dollar spent on the cannabis industry leads to at least two dollars spent elsewhere.