For several decades during the Cold War, British government scientists carried out a covert program testing how biological agents might spread through crowded public spaces, and the London Underground became one of their test sites. Researchers linked to the Microbiological Research Establishment at Porton Down dropped small containers of bacterial spores from moving trains and released organisms in station tunnels, then tracked how far the material travelled and how long it lingered in the air. None of the substances used was actual biological weapons. They were simulants, organisms chosen to behave in ways researchers believed a real biological agent might, allowing them to study dispersal patterns without exposing the public to anything considered genuinely lethal. Ordinary commuters riding those trains had no idea any of it was happening.
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