Over a hundred years ago, a few towns existed in the area that lived off the coal being mined in that region. All that's left today are the mines, and a Cemetery.
Some of the mines are open for guided tours. The visitor center is located in an underground chamber, where it was easily 20 degrees cooler than outside! If you're in the East Bay, a trip is highly recomended by moi. Plus, I found the cleanest pit toilets I have ever seen (or smelled) in my life.