"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" -- Douglas Adams

Fairy rings are incredibly cool, and the causes of them are still not fully known. A commonly acceped theory is that a fairy ring grow from a single spore, what we think of as a mushroom is only part of the story, that's jsut the fruiting bodies. Most of the mushroom is a network of mycelia in the ground. Then when a rainfall or similar comes, mushrooms start popping up in a circular pattern. I'm only a biology student and I haven't had time to check for good sources, I'm jsut gonig by memory here, so if I've misrepresented or misunderstood the fact on the matter, please correct me.