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."

Marco held up a hand. "Wait. Fresh scratches means someone's still down here." He clicked off his headlamp. We stood in the dark, listening over the roar of the falls. Then we heard it—boots on stone, somewhere deeper in the cave. Coming our way.