A fairy ring marks the edge of an underground growth of fungus, called mycelium. This subterranean body spreads slowly outward, emitting chemicals ahead of it that convert organic matter to food usable by the fungus. When the time is right, the mycelium shoots up mushrooms at its outer edge. Like flowers on plants, they distribute spores that will create future fungi and, perhaps, fairy rings.
That’s the scientific explanation. Folklorists tell more colorful tales of fairies and elves, dancing in circles, wearing down the grass and sparking toadstools to sprout. If you weren’t careful and stepped inside a fairy ring, you might be transported into another world.
More fantastic than folklore is fact, however. There’s a fairy ring in France that’s a half-mile wide and said to be 700 years old!