unstable basement with many caves making the creation of any subway a living hell.

That isn't a problem in L.A., but there are areas where they had to build trains above ground rather than underground because parts of the city are floating on beds of tar (hence the La Brea Tar Pits). Believe it or not, there are oil wells in Los Angeles. They're hidden inside building facades.

https://lamag.com/lahistory/hidden-oil-wells