One candidate for the doomsday device is a huge underground cache of so-called salted nukes, preferably humongous hydrogen bombs surrounded by massive amounts of cobalt. Physicist Leo Szilard argued already in 1950 that this could kill everyone on Earth: the hydrogen bomb explosions would render the cobalt radioactive and blow it into the stratosphere, and its five-year half-life is long enough for it to settle all across Earth (especially if twin doomsday devices were placed in opposite hemispheres), but short enough to cause lethal radiation intensity. Media reports suggest that cobalt bombs
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