About the book: 1972 / A group of professional detectives challenges Nancy to solve a mystery that they could not solve: to find a priceless message hidden by a missionary centuries ago in a hollow oak tree in Illinois. While searching the woods for an ancient tree, Nancy and her friends live with a group of young archaeologists who are excavating prehistoric Indian mounds on a nearby farm. An invisible enemy is stalking Nancy and intimidating everyone involved in the excavation. A young detective follows a dangerous enemy to a criminal cave and is threatened when she discovers an unusual treasure. The way Nancy solves complex mysteries with the help of clues will delight all readers.