Clue At The Bottom Of The Lake

Twelve-year-old Jeff Bridger catches a shadowy figure dumping a bundle into the lake late one night, and he and his younger brother David can't resist investigating with their cousin Claire. Their search leads them to Lost Island — off-limits until Jeff's twelfth birthday and never to be explored alone — where the local Legend of Robber's Cave suddenly seems a lot less like a story.

What they find is a duffel bag full of stolen artifacts, dragging the Cabin Creek kids into a real mystery with a real thief still on the loose. The second entry in Kristiana Gregory's Cabin Creek Mysteries series for young readers.