On a warm September afternoon in 1981, 58-year-old Polly Melton was spending time in one of her favorite places on Earth: the Great Smoky Mountains National Park in Tennessee. She went out for an easy hike she’d done dozens of times before with a couple of friends while her husband Bob stayed back at their camper nearby. But strangely, she disappeared on the way back to the campground, and was never seen again.