The Telegraph Buoy
Pranks are an essential component of military culture, and this is true regardless of which branch of the armed forces one serves in. Submarines are not any different from any other kind of vessel in this regard. One of the more amusing tricks that the newbies have to put up with is being asked to retrieve the mail buoy.
In order to retrieve a mail sack from a buoy, new sailors on the submarine are required to don foul weather gear and approach the captain in order to request permission to climb onto the buoy. The request cannot be granted since there is no such thing as a mail buoy in existence.