r documenting voyages to the most extreme landscapes on earth. In Limits of the Known, he reflects on humanity's--and his own--relationship to exploration and extreme risk. Part memoir and part history, this book tries to make sense of why so many have committed their lives to the desperate pursuit of adventure. What compelled Eric Shipton to return, five times, to the ridges of Mt. Everest, plotting the mountain's most treacherous territory years before Hillary and Tenzing's famous ascent? What drove Bill Stone to dive 3,000 feet underground into North America's deepest cave? And what is the future of adventure in a world we have mapped and trodden from end to end? In the wake of his diagnosis with throat cancer, Roberts seeks answers with new urgency and "penetrating self-analysis" (Booklist).