It's the cradle of Shangri-la, and one of the deepest river gorges on earth. It's a fortress guarding sacred waterfalls, and a cauldron of savage whitewater and unrunnable rapids. In the chill of the Himalayan winter, seven world-class kayakers led a massive expedition into the shadowy realm of Tibet's Tsangpo River , and launched their boats down its roaring throat. They were either going to die—or emerge transformed.