Sourdough is pretty interesting, and is the way bread was made for most of human history.

Writing in the Encyclopedia of Food Microbiology M.G. Gaenzle writes "The origins of bread-making are so ancient that everything said about them must be pure speculation. One of the oldest sourdough breads dates from 3700 BCE and was excavated in Switzerland, but the origin of sourdough fermentation likely relates to the origin of agriculture in the Fertile Crescent several thousand years earlier... Bread production relied on the use of sourdough as a leavening agent for most of human history; the use of baker's yeast as a leavening agent dates back less than 150 years."[63] Sourdough remained the usual form of leavening down into the European Middle Ages until being replaced by barm from the beer brewing process, and then later purpose-cultured yeast.

What I'm actually doing isn't very exciting.

I am taking a mixture of water and flour in a 1:1 ratio, and letting it set out in my kitchen. I start off with a small amount, and mix in an equal part every day (doubling it). I'm banking on the fact that the wild yeasts and bacteria in my kitchen will take up residence and form a stable community.

If they take hold, in about a week I should have my target volume of starter, and the community should be strong enough that the bacteria and yeast will fight off anything dangerous for me to eat and I'm good to start baking. Then I'll do my routine of feeding the starter and doubling it, and then take half of the starter and use it for the day's baking.

Fermentation is a great thing.