Note: Do not rely on this information. It is very old.
Fairy RingsFairy Rings, circles of dark-green coarse grass in lawns and pastures produced by the centrifugal growth of the spawn or "mycelium" of various kinds of fungi. The fairy-ring champignon (Marasmius oreades) and the St. George's mushroom (Agaricus gambosus), both valuable as food, are among the commonest species that grow in this manner. The soil within the ring is impoverished, that immediately beneath the ring being temporarily enriched by the decaying crop of fungi. The mycelium dying at its centre while it increases centrifugally, the rings increase in each successive season. Besides the popular explanation that they are caused by the elves dancing in a ring, there have been various other attempts to account for them.
Click for more about Fairy