Fairy rings or what?

[Vimto]

I have often seen both on the telly and in reality, especially on golf courses, circles in cut grassland that are anything from a few feet to maybe thirty or more feet in diameter. Are these caused by the artificial fertilizing of the grassland or by Mother nature? Please advise.

[Buenchico]

Fungi:
http://en.wikipedia.org/wiki/Fairy_ring

Chris