A friend of mine gave me a fantastic book on baking a few years back; among the things it described was how to make sour dough from scratch. The process sounds very similar to what lhun_dweller describes. I think it might even have used the phrase 'wild yeast'. I tried it and it was great fun, but it also felt rather wasteful; you kept doubling the size of your sour dough every day, and then throwing most of it away every four days, for four weeks. I was very pleased with the taste of the bread, even if like lhun_dweller describes, it didn't rise as much as I expected.