A fairy ring is a large colony of fungus which has developed in such a way that when the fungus puts out fruiting bodies, also known as mushrooms, they appear in the form of a ring. Each year, the fairy ring gets larger, with some fairy rings being quite large, and hundreds of years old. Fairy rings look truly distinctive and unique, and many cultures include fairy rings in their folklore, thanks to the fact that they can seem a bit mysterious.
When a fairy ring is not putting out mushrooms, it may still be visible. The grass inside many fairy rings is a different color, as the mycelium of the fungus typically consumes a large amount of the available nutrients, so the grass or plants inside a fairy ring can die off or yellow. The perimeter of the fairy ring, on the other hand, may receive extra nutrition from the mycelium as it prepares to fruit, and as a result it can appear much greener and darker.
Fairy rings are probably most common in woodland areas, but they can also be found in open fields and meadows. At least 40 species of fungus are known to produce fairy rings, and there may be more. No one is quite sure why fairy rings form, but they typically consist of one large fungus which has simply grown outwards in all directions, probably as a result of ample nutrients in the ground to feed the fungus. The spreading growth may be caused by a desire to seek out more nutrients as the mycelium depletes them. People with lawns sometimes find fairy rings frustrating, as they are impossible to control, and they are very noticeable.
You may also hear the fairy ring referred to as an elf ring or pixie ring. These common names for this interesting phenomenon all reference folk beliefs about the fairy ring; many cultures believe that fairy rings are used as a dancing ground by fairies. This would also explain the dead grass in the middle, which would presumably be trampled by little feet as fairies danced.
Some people think that fairy rings are lucky, linking them with more general beliefs about fairies being good omens. Others found them unlucky historically, fearing that the fairies were actually works of the devil who might try to entice people to the dark side. Modern observers often simply find them interesting and pretty to look at, especially when extremely large and well developed. It can be fun to visit a fairy ring every year to watch it grow, as it will continue to spread unless the soil is radically disturbed.