Crouched in the darkness, two field technicians squint at a laptop, hoping to catch a glimpse of California's most elusive predator.

"Possum. Possum. Possum," Richard Pickens says, flipping through photos. "Oh—fox."

Pickens is studying images from a camera stashed a few miles east of San Jose, California, among bay trees and meadows of thistle and stinging nettle. Scientists guess that about a half dozen adult mountain lions, also called pumas or cougars, prowl these hills. But tonight, the golden animals are living up to one of their many nicknames and slipping through the forest like true ghost cats. (Read about cougars in National Geographic magazine.)

That isn't always the case. As California's concrete jungles creep continually outward, more and more cats are