TIL that O’Neill cylinders are space habitats proposed by physicist Gerard K. O’Neill. They consist of two rotating cylinders that generate artificial gravity and contain cities, farmland, and landscapes, with sunlight directed by mirrors, creating a livable environment for long-term habitation.

https://en.wikipedia.org/wiki/O%27Neill_cylinder
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https://www.reddit.com/r/todayilearned/comments/1g793mr/til_that_oneill_cylinders_are_space_habitats/