Shaft? You're Damn Right!

Q:When is a summerhouse not a summerhouse?
A:When it's a ventilation shaft.
Sorry, not as good as "when is a door not a door?", but at least it's accurate.
This was built in 1976, for what was then to be called the Fleet Line, to disguise one of the ventilation shafts at Regent's Park tube station, thereby preserving the Georgian splendour of the area.
At the last minute the Fleet Line became the Jubilee Line, to mark the Queen's Silver Jubilee in '77.