The Soviet Doomsday Ship
In the crazed height of the cold war, a proposal crossed Nikita Khrushchev's desk that supposedly ordered - or had already ordered - it's not wholly clear - the construction of a gigantic doomsday ship. This beast, to move just off the Russian coast at all times, was a cargo ship to loaded with an immense amount of fissile material, and literally hundreds of hydrogen bombs capable of producing an explosion so vast it would render the planet uninhabitable. The scariest part? The explosives would have been automated, rigged to a system that detected atmospheric radiation. That way, even if the Soviet Union was taken off guard by nuclear attack, the automatic system would get revenge on the world by cracking the planet's crust, triggering massive earthquakes, and filling the world's atmosphere with deadly radioactive material. According to the story, Khrushchev's scientists persuaded him it was a bad idea when they pointed out that they could make a system just as secure that used a human element of control - but that's no guarantee that Khrushchev didn't build something just as insane. After all, he's the one who had Tsar Bomba built.
Whether you believe the story or not, well, the fact that this technology is perfectly feasible should scare you enough.