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.

Inside, our lights found shelves of dynamite crates, rusted tools, and a single iron strongbox on a table. Marco lifted the lid. Gold coins glinted, plus a leather journal. "Hennig was telling the truth," he breathed. Then the door behind us creaked—and slowly began to swing shut on its own.

I jammed my boot in the gap just before the door slammed. "Grab the box and go!" Marco scooped up the journal and coins. We squeezed out and the heavy door boomed shut behind us. Back at the cave mouth, we stood gasping in the spray. We'd done it—and we had proof Hennig's vault was real.