The brilliant scientists who created nuclear weapons were appalled by what they had made possible. Nuclear war. During the Cold War, they saw humanity inching steadily toward self-annihilation. They started a movement among themselves to lobby for nuclear sanity. They used the image of a clock showing minutes to midnight to make plain the imminence of existential risk. It was on the cover of every issue of their monthly magazine, the Bulletin of the Atomic Scientists. The minute hand moved back a bit when disarmament negotiations showed progress. It moved forward when tensions rose, or nuclear sabers rattled. We called it the Doomsday Clock. Clearly, humanity was in uncharted territory, master of its own fate and hostage to its worst instincts.