Folklore tells us that these rings are caused by fairies, pixies, or elves dancing in a circle.
To step into a fairy ring is to risk never being able to step back out into our world again. You might find yourself in the Land of Faerie - stuck there, bidden to dance until you die from exhaustion. Or you may become enchanted and marry a fairy. Most legends say that fairy rings are dangerous places and are best avoided. My dog, however, made it back out of this Fairy ring just fine.
In reality, the rings are caused by an underground fungus called mycelium. You can learn more about fairy rings, their causes, and the folklore behind them, here.
It's either fairies or fungus that really like the soccer fields in the park next door. Maybe on the next moonlit night I'll go and make sure that it isn't the former.
If you would like to read a short story with fairies in it, follow this link to a story called The Fairy's Tale.