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 knelt and studied the scratches with my light. They weren't random. Someone had carved an arrow into the steel, pointing down and to the left, away from the vault door. Priya frowned. "A warning? Or a trail?" There was a second passage I hadn't noticed, half-hidden behind a curtain of dripping rock.

We followed the arrow into the second passage. It narrowed until we crawled on our hands and knees. Then it opened into a wide chamber, and our lights swept across it. Old mining carts. A collapsed tunnel. And a second steel door, this one wide open, with a lantern still glowing inside.