Doomsday device
A doomsday device is a hypothetical construction – usually a weapon or weapons system – which could destroy all life on a planet, particularly Earth, or destroy the planet itself, bringing "doomsday", a term used for the end of planet Earth. Most hypothetical constructions rely on hydrogen bombs being made arbitrarily large, assuming there are no concerns about delivering them to a target (see Teller–Ulam design) or that they can be "salted" with materials designed to create long-lasting and hazardous fallout (e.g., a cobalt bomb).