Pennsylvania has [ 200,000 abandoned oil wells ]( http://www.theguardian.com/environment/2014/sep/18/pennsylvania-abandoned-fracking-wells-methane-leaks-hidden ) , and when fracking is done near them, they become channels for leaking methane.
Ссылка: https://stallman.org/archives/2014-jul-oct.html#20_September_2014_%28Abandoned_oil_wells%29