@mattblaze London's Tube (and other tunnels) system has vents hiding in plain sight all over the city. Some are old stations, some are hiding in other structures, yet more are custom built. Apart from maybe a discrete TfL sign, they're remarkable in that they're almost all unremarkable.One example is the triumphal Wellington Arch at Hyde Park Corner - one side of it is the ventilation shaft for the road underpass connecting Knightsbridge to Piccadilly.
