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 vault floor was flooded with ankle-deep black water. Our lights caught something floating—a torn backpack, soaked and abandoned. "That's recent," I said, my voice tight. From somewhere under the water came a low, hollow gurgle. We all froze.