A fairy ring is a naturally occurring circle of mushrooms and is a source of a lot of folklore, especially in western European cultures. Some associate them with witches or the devil, while others associate them with elves and fairies. English, Scandinavian, and Celtic traditions say that they're a result of fairies or elves dancing in a circle.

There's a lot of superstition about entering one of these - you may anger the fairies and end up with a curse.