General wear on cover you'd expect from a vintage book. Yellowing and age spots on the pages. Otherwise clean pages. Published 1962. 'Amateur sleuths Frank and Joe Hardy investigate a counterfeit operation while their detective father is connected to a secret case involving national security. An old mill on the outskirts of town appears to hold the key to solving both crimes. Determined to learn the secret of the old mill, Frank and Joe devise a clever scheme to gain entrance, only to find themselves trapped in an underground tunnel.'