" was an atomic bomb
conceived by nuclear physicist Leo Szilard
origin) in February, 1950 that could kill (nearly) everybody on Earth.
Though just an idea, not a serious proposal, it is still distressingly real.
It is essentially a fission-fusion-fission nuclear weapon where the third (fission) stage is replaced by a non-fissionable "salting" isotope, therefore producing a so-called "salted" bomb.
Though some other isotopes (such as Zinc-64) could suffice, Leo's "Doomsday Device" used Cobalt-59, which would be the most practical choice. It would capture the neutrons from the secondary (fusion) stage, becoming radioactive Cobalt-60, with a half-life of 5.26 years - too long to wait out for decay in the shelters, but short enough to produce considerable radiation intensity.
Fortunately, no such bombs have ever been known to be built nor tested, and, hopefully, never will.
In Stanley Kubrick's classic movie "Dr. Strangelove", the Soviets have built an auto-magical "Doomsday Device" that goes off in case of a nuclear attack on USSR.