The Secret of the Forgotten Cave — Carolyn Keene

Nancy goes to Connecticut to visit George's Aunt Elizabeth -- and lands right in the middle of a controversy. George's aunt, among others, proposes a bicycle path instead of widening a dangerously winding country road, in order to save a rare species of bat. But a threatening phone call, a sinister warning hanging in the doorway, and a sabotaged car convince Nancy that much more than the bats are at stake.