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.

Marco reached for the floating backpack. The second he touched it, the floor dropped away—a flooded shaft. He went under. I grabbed his jacket and Priya grabbed me, and we hauled with everything we had. He came up coughing. "Out," he choked. "Now." We sloshed back to the door and didn't stop running.