|A fairy ring growing on a lawn in Queensland|
|An illustration from 1880 of the dancing fairy myth|
In fact, the truth is that they are a naturally occurring phenomenon that is caused by the natural growth process of certain Basidiomycetes (the class of fungi that produce mushrooms). Fungal colonies can be quite large, and mushrooms will only sprout from the edges of colonies. The older the colony, the larger the fairy ring that is formed will be. It is not unheard of for a ring to be as large as two hundred feet in diameter. Large fairy ring may be quite old, with growth rates ranging from about three to nineteen inches per year. If the average growth rate is three inches a year, a two hundred foot ring would be eight hundred years old!
I do find it interesting that fairy rings are considered a "turf disease." I actually find them quite attractive, and they usually won't kill the grass. In fact, one common characteristic of the rings is taller grass produced by the extra nutrients that the fungus releases as it feeds on the soil. This means that a common first sign for a fairy ring is not the mushrooms themselves, but actually a ring of taller than average grass. There are several near where I live, but the mushrooms are dieing or falling over and are thus not very photogenic.
Source is Colorado State University. Images are from Wikimedia Commons and are copyright free: one, two