Myths, folklore, and symbolism
Fairy Rings: Circle dancing (carol) was the norm in medieval society, but later English dances were so different that dancing in a ring came to seem not merely old-fashioned or childish but uncanny, associated with fairies or witches. When circles of lush, dark, green grass were seen in meadows, they were said to mark the place where fairies had danced, and anybody who set foot in one risked being carried off. Such rings are caused by underground fungi, which at times produce visible toadstools; they persist for years, getting steadily larger, and grazing animals avoid them. No serious belief now attaches to them; at most, a vague idea that it is unlucky to step into one, or, contrariwise, that one can make a wish. (Oxford Dictionary of English Folklore)
Do you know of fairy ring folklore from other countries?