Death Mode by Thomas Black

Seventy-five years ago, John Dillinger stashed silver and gold coins in the town of Elkinsville, Indiana. His great-grandson wants the lost loot. Two catches. First, Elkinsville is now a forgotten town, sixty feet below the surface of a reservoir. Easy, blow the dam holding the water. Second, a fiery speleologist, Dr. Jenna Longstreth, has discovered his plan. Not so easy, as he finds out.