Fairy Rings

A fairy ring, also known as fairy circle, is a naturally occurring ring or arc of mushrooms.
Fairy rings typically have a ring of dead grass which is bordered on both sides by a band of stimulated grass growth.

Fungal fruiting bodies (mushrooms or toadstools) may be found associated with these rings.
You will see them appear is late summer to early autumn, and they are more likely to appear after wet weather.