Wikipedia page on this place: https://en.wikipedia.org/wiki/Underground_City_(Beijing) https://en.wikipedia.org/wiki/Underground_City_(Beijing) Wikipedia page

Wikipedia page on this place: https://en.wikipedia.org/wiki/Underground_City_(Beijing) https://en.wikipedia.org/wiki/Underground_City_(Beijing) Wikipedia page states that is 85 square km. Also another much longer and detailed Vice article of it: https://www.vice.com/en_uk/article/nnmk8b/chairman-maos-underground-city https://www.vice.com/en_uk/article/nnmk8b/chairman-maos-unde...