In California, abandoned mines are everywhere

For urban spelunkers and infiltrators, California is a recreational heaven. For people just minding their own business -- and building new homes on once-remote inland foothills -- it can be a holey Hell. Literally: after more than 100 years of heavy-mining operations, from the '49ers onward, California's easily-accessed gold and silver have been replaced by at least 40,000 abandoned mines, and many more ancillary mine structures like tunnels and shafts.

While these can be fascinating places -- I admit that I have a tough time staying out of dark holes in the sides of hills when I come across them; the idea of finding some 100-year-old miner's sardine can, still moldering where he left it, is just too cool -- they're also dangerous. In the last few years, homes have collapsed into crumbling mine shafts, swallowed trees and claimed several lives annually. In fact, there are so many mines that California can't even count them: "We've estimated that it would take 26 years for us to complete an inventory of all abandoned mines in the state," says the mining office's assistant director, Doug Craig.

More photos: