thumb|upright|Many hypothetical doomsday devices are based on [[Salted bomb|salted hydrogen bombs creating large amounts of nuclear fallout. Above, the Castle Romeo nuclear test (yield 11megatons) at Bikini Atoll of the Marshall Islands, conducted on a barge moored in the middle of the crater created from the Castle Bravo test and the first nuclear test conducted on a vessel (March26, 1954).]]
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).
Doomsday devices and the nuclear holocaust they bring about have been present in literature and art especially in the 20th century, when advances in science and technology made world destruction (or at least the eradication of all human life) a credible scenario. Many classics in the genre of science fiction take up the theme in this respect. The term "doomsday machine" itself is attested from 1960, but the alliterative "doomsday device" has since become the more popular phrase.
History
Since the 1954 Castle Bravo thermonuclear weapon test demonstrated the feasibility of making arbitrarily large nuclear devices which could cover vast areas with radioactive fallout by rendering anything around them intensely radioactive, nuclear weapons theorists such as Leo Szilard conceived of a doomsday machine, a massive thermonuclear device surrounded by hundreds of tons of cobalt which, when detonated, would create massive amounts of Cobalt-60, rendering most of the Earth too radioactive to support life. RAND strategist Herman Kahn postulated that Soviet or US nuclear decision makers might choose to build a doomsday machine that would consist of a computer linked to a stockpile of hydrogen bombs, programmed to detonate them all and bathe the planet in nuclear fallout at the signal of an impending nuclear attack from another nation.
The doomsday device's theoretical ability to deter a nuclear attack is that it would go off automatically without human aid and despite human intervention. Kahn conceded that some planners might see "doomsday machines" as providing a highly credible threat that would dissuade attackers and avoid the dangerous game of brinkmanship caused by the massive retaliation concept which governed US-Soviet nuclear relations in the mid-1950s. However, in his discussion of doomsday machines, Kahn raises the problem of a nuclear-armed Nth country triggering a doomsday machine, and states that he didn't advocate that the US acquire a doomsday machine.
The Dead Hand (or "Perimeter") system built by the Soviet Union during the Cold War has been called a "doomsday machine" due to its fail-deadly design and nuclear capabilities.
In current time in the media the USS Alaska (SSBN-732) has been referred to as a "doomsday weapon", because of its role as an Ohio-class nuclear-powered ballistic missile submarine. It is designed to serve as the ultimate deterrent in the U.S. nuclear triad. The term describes the ship's ability to "deliver catastrophic nuclear strikes from a hidden, near-invulnerable position." In 1957, the apocalyptic novel On The Beach was written by the notable British-Australian writer/aeronautical engineer Nevil Shute Norway. The story talks about the disastrous event of a massive nuclear World War III using cobalt bombs that slowly poison the atmosphere of the entire earth, and how the remaining inhabitants in Australia wait for their end. Adapted into a film with the same name it was praised for its "antibomb" story.
Another well-known example is in the film Dr. Strangelove (1964), where a doomsday device, based on Szilard and Kahn's ideas, is triggered by an incompletely aborted American attack and all life on Earth is extinguished. This includes the missile launch system in the movie WarGames (1983), control of which has been handed entirely to a computer, and Skynet's nigh-destruction of the human race in The Terminator (1984).