In the summer of 1976, a mysterious and seemingly very powerful signal began infiltrating radio receivers around the globe. It has a signature 'knocking' sound when heard, and because the origin of this powerful signal was somewhere in the Soviet Union, the signal was given the nickname Russian Woodpecker. Many amateur radio listeners believed it to be part of the Soviet Unions over-the-horizon radar, however the Soviets denied they had anything to do with such signal. Between 1976 and 1989, the signal would come and go on many occasions and was most prominent on the shortwave radio bands. It was not until the end of the Cold War that the Russians admitted these radar pings were indeed that of "Duga", an advanced over-the-horizon radar system.