If you're into GPS you may want to check out http://www.calsign.com/mining/counties.htm -- This gives you innumerable GPS coordinates throughout the state which can be put up in Google Maps and downloaded into your GPS device. Some of the sites it covers
include ghost towns, town sites, cemeteries, camps, old names of towns that have been changed or absorbed into other towns. Historical names, ruins, old mines, airports with 1173 listings for Santa Clara County, 939 San Mateo, 488 for San Francisco County, etc...All 58 CA counties have large listings. All of them historical. I've tracked down a few of these sites in Santa Clara county and been out to them. Only 3 have been accessable; the problem mainly being the Bay Area has become so highly urbanized. If it's not been turned into an asphalt jungle of some sort, it's got some sort of historical marker which puts it off limits. Even discounting GPS, my suggestion is if you want to dig anything other than clad or the occasional piece of jewelry is to get to the outlying counties where the smaller towns are with less urbanization. If you're not into research I agree with Willee, join a club. There are quite a few in the Bay Area. They can help you get on the inside track. Findmall also has a Maps & Mapping Forum here http://www.findmall.com/list.php?69