In 1950 Leo Szilard warned the world that a single device capable of annihilating all life on Earth was theoretically possible.
Leo Szilard was one of the most influential figures in the early history of nuclear weapons, especially when it came to thinking the previously unthinkable. He was the one who worked out the possibility of a nuclear chain reaction in the first place. He was the first one to patent a nuclear fission reactor (although it was never built and his design would never have worked). The letter that warned the US president of the possibility of nuclear weapons, the letter which kickstarted the Manhattan Project, was written by Szilard and signed by Albert Einstein.
So, when he made a warning, everyone listened. In 1950, exactly 69 years before this website began, he appeared on a radio show and said the following:
Interviewer: You mean, Szilard, that if you exploded five hundred tons of heavy hydrogen and then permitted those neutrons to be absorbed by another element to produce a radioactive substance, all people on earth could be killed under the circumstances?
Szilard: If this is a long-lived element which gradually settles out, as it will in a few years, forming a dust layer on the surface of the Earth, everyone would be killed.The Facts about the Hydrogen Bomb
What Szilard was talking about here was the possibility of a salted bomb. Nuclear fallout actually disperses pretty quickly, but if you surrounded a bomb with the right kind of material you can extend the range and duration of the fallout dramatically. Surround a hydrogen bomb with enough cobalt, for example, and the neutrons from the explosion would turn that cobalt into a radioactive isotope called cobalt-60. Cobalt-60 is lethal for years rather than days, giving it enough time to spread around the world and so “salting” the whole Earth. Such a scenario would pretty much wrap things up for the human race.
This was the first time someone actually outlined a plausible doomsday device. At the time, Szilard’s claim was attacked as impossible and alarmist. Later research has corroborated it in theory. Fortunately, no-one that we know of has ever built this kind of salted bomb – its only appearances have been in fiction, such as Stanley Kubrick’s classic film Dr. Strangelove, or How I Learned to Stop Worrying and Love the Bomb.