Some people might only dream of adventure, but for others, there’s no other option but to explore the most extreme places on Earth. From the deepest caves to rough oceans, from the North Pole to dizzying heights on a high wire  — what drives adventurers to constantly push to the brink of human endurance? In this hour, TED speakers share their experiences of going to the edge of our world. Arctic explorer Ben Saunders recounts his harrowing solo ski trek to the North Pole. Engineer and daredevil caver Bill Stone pushes the frontier to the remotest depths of the Earth. Roz Savage quit her high-powered job to become an ocean rower. High-wire artist Philippe Petit tells the amazing story of how he walked between the Twin Towers.