A mysterious Russian radio signal at 4625kHz has been broadcasting since the 70s. Mostly it consists of a intermittent buzzing noise, but a 2-3 times a decade it stops and a voice can be heard. These interruptions have included '"Command 135 initiated", a playing of Swan Lake, and some overheard background conversations.
Possible explanations are that it's a simple emergency transmission frequency, or it could be part of Russia's "dead man switch", which automatically fires off nukes if Russia is otherwise crippled. It's also been postulated that it could be part of a system for measuring changes in the ionsphere: