Mystery of The Vanishing Skinny-Dippers

Book 1: Four teenagers go camping in the woods outside of town. They spot some skinny-dippers in the nearby lake, but the nude swimmers see them on the shore, swim across the lake, and vanish. Suddenly, the camping trip has turned into a mystery. Follow the four teenagers around, as they search all around the lake, trying to determine how and where to, the nude swimmers vanished from the valley. (Naturist Fiction)