Every once in a while, people encounter things on their radio, TV, or interstellar wave detectors that no one can explain. Is it a secret military message? A prankster hacking just to see if it can be done? Aliens trying to contact us? Cracked has five cases that still haven't been settled. Take, for example, the case of UVB-76.
It is an irritating, electronic noise, not unlike the sound of a truck horn played through a cheese grater. It is broadcast over a certain frequency, constantly, and has been since at least 1982. But the weird part isn't the tone, but what happens when it stops.
In its 20-something year run, the sound has been interrupted only three times, the earliest known time being Christmas Eve in 1997. Each time a voice comes on and lists several Russian names and numbers before returning to the foghorn. The most recent occurrence was 2006, a mere three years before the time of this writing. It is clearly becoming more active after remaining quiet during the Cold War.