The Soviet Doomsday Machine
They built it in the 1980s, and it's still around. Nicholas Thompson explores the paradoxical nature of nuclear weaponry:
[O]ne obvious question that people ask when they learn about this system is “Why didn’t the Soviets tell us?” As we all know from Dr. Strangelove, the point of a Doomsday Machine is to convince the other side in a conflict that attack is futile. The answer to that question is the most interesting thing I learned in my research. Yes, the Soviets were extremely secretive; and, yes, they were worried that, if they told us, we could disable it. But the more interesting reason is that they also built the system to deter themselves.