The Jolly Roger, a sign of victory, is flown each time a submarine returns home from a successful wartime patrol.
At the beginning of the 20th century, submarines started off as being quite unpopular. One of the most vocal opponents at the time, Admiral Arthur Wilson, described submarines as "underhand, underwater and damned un-English", and declared that any Submariner captured during wartime should be 'hanged as a pirate.'
Subsequently, when the first submarine, HM E9, sank an enemy warship in WWI, she returned triumphantly into the harbour with her periscope raised and a black flag with the skull and crossbones flying proudly - a cheeky 'dig' at Admiral Wilson.
The march The Jolly Roger was the winning entry for a competition to find a march to mark the 2001 Centenary. It has since been adopted as the official march of HM Submarines.