On Oregon’s most popular mountain, Brent McGregor and Eddy Cartaya made a remarkable discovery. After rapelling 15 stories into a hole in the Sandy glacier, they encountered a mile of caves and tunnels laced with waterfalls, whitewater, towering ice sculptures and otherworldly light. It’s the largest glacier cave system outside of Alaska. But the caves are disappearing fast, as the northwest’s glaciers retreat in the face of climate change.