We put in our first morning five miles from the Argentine border, just above Inferno canyon. The morning was warm, hot even, but the water was cold, so we donned wetsuits. For the next two hours, our 18-foot inflatable raft thundered down through a series of rapids - Inferno, Purgatorio, and Danza de los Angeles ("Dance of the Angels"). On our final rapid, a violent Class Five named Escala De Jacoba ("Jacob's Ladder"), our boat was sucked into a hole with such force that Eric, our guide, was thrown from the back of the boat. Amazingly, we all kept our heads, letting Eric's training regimen take over instead. We paddled the boat into an eddy and waited while Eric swam to shore. "It's 100 percent adventure, and yet it feels 100 percent safe," says Kim Carter, a businessman from Bermuda and one of my paddlemates, while we waited in the eddy for Eric to rejoin us.

We pushed off again, the day's rapids behind us, and drifted leisurely for the rest of the afternoon until we reached the cave camp. The second of Earth River's camps on the Fu, the cave camp is a natural, adult-size playground. Like a grown-up version of a tree fort, it's the type of never-never land that would make Peter Pan and the Lost Boys jealous. Located in the rocks above Zeta, the river's meanest rapid, and one that for safety's sake we chose to avoid, the cave camp takes its name from the huge granite overhang that serves as the main sleeping area. Aside from the novelty of sleeping in a cave, the camp also boasts miles of trails, smaller private stone shelters, natural stone hot tubs, tunnels, alien rock formations, and beautiful crystalline pools with huge German brown trout in plain sight. Not to mention a small mountain climb and ropes course designed by renowned rope course designer Paul Wolf.

I opt to skip sleeping in the cave with my fellow travelers and instead set my sleeping bag up on a flat rock directly above Zeta. I sit staring up at the biggest full moon I've ever seen. When I finally tire of counting craters, I close my eyes and drift off, the booming Fu singing me to sleep.       

Stephen Mahan is a New York-based travel and adventure photographer whose work has appeared in Condé Nast Traveler and National Geographic Adventurer magazines.