This is something I always worried about after buying my house in SoCal. When I purchased the house it came up in the closing disclosures that there was a "capped" oil well within 500 feet of our house. I search every where for it. Asked neighbors and everyone was clueless to it's where being. Needless to say I never found it but did have the air in our house checked for methane as well as radon which were at normal levels.

The LA basin had a lot wells especially in the South Bay [1]. I could totally see this being a problem especially since they were capped so long ago.

1.) http://blogs.dailybreeze.com/history/2014/10/18/torrance-bec...