The rangers found the corpse shortly past twelve o’clock on a warm day in August 2010. They were deep in Dark Canyon, on the 818 side of the Santa Monica Mountains, inspecting a marijuana farm that had allegedly been run by a Mexican cartel.

They were familiar with the farm. Just over a year earlier, the L.A. County Sheriff’s Department had flown over Dark Canyon and spotted the “grow,” along with a pair of others in the Malibu region—swaths of ganja that had been planted in spring and left to mature until the late summer harvest. After the flyover, the farms were raided. As expected, the growers were absent, and a thousand plants were uprooted in Dark Canyon alone.

Dark Canyon is a sensible place for a pot farm. Located near Calabasas, it’s less than eight miles from the 101 freeway, yet it’s rugged and seldom traveled. Hemmed in by private and federal land, it begins at the top of Piuma Road and descends from south to north, with a boulder-strewn creek—Dark Creek—running the length of the lush canyon bottom. Besides a blip of the Santa Monica Mountains Backbone Trail that crosses the lower part of the drainage, there are no official footpaths. In the wet season poison oak is unavoidable. Year-round the narrow canyon floor is rife with live oaks and scratchy laurel sumacs. You don’t stroll along Dark Creek, you negotiate it—hopping, climbing, concentrating.

On this summer day the rangers were making sure the pot operation had remained defunct. Some equipment was lying around—hundreds of feet of garden hoses that once siphoned creek water into spigoted PVC lines—but the cannabis was gone. Satisfied that the growers hadn’t replanted, the rangers headed downstream. Negotiating a series of boulders, they detoured through a wide clearing about 60 feet upslope from the creek bed. Then they noticed the skull and, beneath the leaf debris and dirt, a semidecomposed, naked body.

The men radioed State Parks dispatch, which alerted the M...