aramis erak
Legend
Not in San Fran... but some great locations come to mind.
Derinkuyu (as mentioned by others)
Kowloon, the walled city by Hong Kong, where population density was worse than Singapore... https://www.businessinsider.com/kowloon-walled-city-photos-2015-2
https://www.scmp.com/news/hong-kong...thing-else-25-years-hong-kongs-kowloon-walled
The underground city of Seattle... https://www.atlasobscura.com/places/seattle-underground
A list of undergound cities at https://www.history.com/news/8-mysterious-underground-cities
Derinkuyu (as mentioned by others)
Kowloon, the walled city by Hong Kong, where population density was worse than Singapore... https://www.businessinsider.com/kowloon-walled-city-photos-2015-2
https://www.scmp.com/news/hong-kong...thing-else-25-years-hong-kongs-kowloon-walled
The underground city of Seattle... https://www.atlasobscura.com/places/seattle-underground
A list of undergound cities at https://www.history.com/news/8-mysterious-underground-cities