Snow, who is fifty-seven, has the dour, keen-eyed look of a large waterbird—natural enemy of snakes. His face is thin and angular, its balding dome crowned by gray bristles. He has long legs, sloping shoulders, and a craning neck. When folded up behind the wheel of a car, as he often is, on his python-hunting trips, he can never seem to get comfortable, bobbling up and down and bending at the waist, squinching his eyes and jerking his head to the side. Yet he keeps his truck at a maddeningly slow pace, eyes fixed on the shoulders of the road.

“I won’t pass up a snake,” he told me as we were driving through the park one afternoon. “I may not be looking with every ounce of my body every minute. But there are search images that get developed: the feel, the look, the body posture, and head shape—all these things together.” The day before, as we were leaving the park, Snow had suddenly swerved his truck around and doubled back down the road. He’d seen a black shape winding across the asphalt, he explained, but by the time he stepped out to catch it, it was gone. “Probably just a water moccasin,” he said—poisonous, but only a fraction of a python’s size.

Snow has spent his entire career with the park service. After earning a master’s degree in environmental sciences from Miami University in Ohio, he took positions at Mt. Rainier and at Theodore Roosevelt National Park, in North Dakota..... The closest big city was Bismarck, three hours away; the closest thing to a social life shooting hoops with the two other park-service employees in the district. In 1988, Snow applied for a post in the Everglades, hoping to find a warmer, more stimulating habitat for his family. Chasing giant snakes wasn’t really what he had in mind.