Nancy Drew Mystery Stories #28: The Clue of the Black Keys book by Carolyn Keene

During an archaeological expedition in Mexico, two professors, the senior Dr. Joshua Pitt and young Terry Scott find a clue to buried treasure. The clue was a cipher carved on a stone tablet. Before the older professor had time to translate it, he and the tablet disappeared! Terry tells Nancy of his suspicions of a Mexican couple, posing as scientists, who vanished the same night as Dr. Pitt. Nancy follows a tangled trail of clues that lead to Florida and Mexico and a secret of antiquity that can only be unlocked by three black keys. This book is the revised text. The plot of the original story (©1951) is similar with minor revisions.