Journalist James Nestor was on assignment in Greece when he saw a man dive into the ocean and surface four minutes later. The man had swum down 300 feet on a single breath of air -- part of a sport called "freediving." Nestor embedded himself with a group of athletes and researchers and discovered a world of competitive freediving where people risk death without oxygen tanks. He also went freediving with dolphins and sharks and rode a handmade submarine toward the ocean floor. Nestor tells us what he uncovered about why people and sea mammals can dive as far as they do. James Nestor will be speaking July 23 (Wednesday) at the Mechanics' Institute at 6pm.