abandoned mines have become a hot topic in government circles because there are so many of them and they're so dangerous and hard to find. the average calculation within california is about 39,000 with a high of nearly 60,000. aside from people falling into vertical shafts covered by brush or rotten wood, homes have caved into subsided mines (one death that i know of a guy in his basement that went into the mine below), and over cave ins have happened on private property in the way of vertical sink holes. also, the chemicals that were used and long buried are percolating either directly into the ground water system or leeching out on the surface into creeks and lakes. i.e. clearlake, california has a problem with mercury i think.... fishing in the lake is legal... but eating the fish is not officially sanctioned by the state due to high levels of heavy metal. the problem is first, nobody knows where they all are... and secondly.... its expensive to deal with them. one idea was to fill vertical shafts with hard foam .... but the political troublemakers screamed about that so i don't know what is next. currently, i've read they just fence them off & if you trespass, well.... you're gonna die.