The "Doomsday Clock" by the Bulletin of the Atomic Scientists measures the likelihood of a global cataclysm by reflecting how vulnerable the world is to catastrophe from nuclear weapons, climate change and new technologies. Midnight represents the apocalypse. Some key dates in the clock's nearly 70-year history:
1953 — The clock comes the closest it ever has to midnight, just two minutes away, after the U.S. and Soviet Union test hydrogen bombs. "The hands of the Clock of Doom have moved again," the Bulletin announces. "Only a few more swings of the pendulum, and, from Moscow to Chicago, atomic explosions will strike midnight for Western civilization."
1981 — The clock moves to four minutes-to-midnight after the Soviet Union invades Afghanistan and U.S. President Jimmy Carter pulls the U.S. from the Olympics in Moscow.
1991 — The clock drops to 17 minutes-to-midnight as the Cold War officially ends and the U.S. and Russia begin making deep cuts to their nuclear arsenals. "The illusion that tens of thousands of nuclear weapons are a guarantor of national security has been stripped away," the Bulletin says.
1998 — The clock moves to nine minutes-to-midnight after India and Pakistan stage nuclear weapons tests.
2015 — The clock moves to three minutes-to-midnight. The bulletin cites "unchecked climate change, global nuclear weapons modernizations, and outsized nuclear weapons arsenals'' that pose "extraordinary and undeniable threats to the continued existence of humanity."
