During the Cold War, the world went MAD, which means "mutually assured destruction." The concept meant that if your nukes were deployed at my country, ours would automatically be deployed at yours. The technology developed for this fatalistic deterrence system is still around today, as far as we know. In the 1980s, the Soviet Union developed a "doomsday device" they called Система «Периметр», or Perimeter System. It came to be known as Dead Hand. But it was a state secret- the rest of the world didn't even know about Dead Hand until 1993, after the fall of the Soviet Union. What kind of deterrence is that?
If you want a doomsday device to function as a way to frighten would-be attackers into not attacking you, they have to know the device exists. Secrecy renders the whole point moot. That is, unless the purpose of the device is not as clear as it would seem.
Dead Hand, it turns out, may not have been primarily a deterrent against the Americans launching a nuclear attack against the Soviets, but rather was a Soviet-built safeguard to prevent themselves from launching a nuclear attack unnecessarily.