Adventure at Black Rock Cave

"Don't go there again!" Chris's father warned. "There's something very strange going on at Black Rock Island."
Chris and Addie are sure of that. For they have seen the two strange men—the tall one with the horrible grin, and the short one with the black, glittering eyes.
Who are these two men? What are they trying to hide?
Chris and Addie are determined to solve the puzzling secret of Black Rock Cave.