Glasswater Falls thundered behind us as we squeezed into the cave mouth. Three headlamps swung across wet limestone. Old Hennig's map promised a miner's vault down here, and there it was: a steel door, factory-gray, slick with damp. But the scratches around the lock looked fresh. Somebody had been here recently. "Guys," Priya whispered, "that's not rust."

I grabbed the door handle and pulled. It groaned open an inch, then stuck. Cold air hissed out, smelling of metal and old dust. "It's not even locked," I said. Marco and Priya leaned in beside me and we heaved together. The door swung wide into the dark.

The room was empty except for muddy bootprints leading to the far wall. Whoever came before us hadn't left through this door. "They went somewhere," Priya said, following the prints. They stopped at a blank stone wall—a dead end. Or so it looked. She pressed it, and the wall shifted.