It was the mask worn by V, the revolutionary leader in the comic book series "V for Vendetta" published in the 1980s.

BBC: V is for what? The meaning of the mask

Once Brompton Road is finished, he explained, he will be turning his attention to the abandoned Aldwych station in central London, which has been used for Tube locations in Hollywood films like V for Vendetta, Atonement, and28 Weeks Later.

BBC: Reawakening the Underground

One of them, we see, included a V for Vendetta mask to underscore the point.

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