Fairy Ring Mushrooms
Underground fungal colonies can radiate outward in all directions and
sometimes form a circular pattern.  Fairy rings, a common name for
this phenomenon,
are most apparent in mowed fields and lawns.  
Field rings is another term, although woodland mushrooms will also
exhibit circular patterns of growth.
Fungi in the soil interact with grasses, consuming organic material in dead
grass and by exchanging nutrients with live grass.  The above fairy rings are
the work of
Agaricus campestris, a common meadow mushroom.  Note lush
grass (first growth of spring before being mowed) were the mushrooms
appeared the previous autumn.  
October 2008
October 2008
April 2009
April 2009
Google Earth images showing fairy rings.  Left to right:  park behind city hall, Hammond, IN; field
near urban development, Indianapolis, IN; farm house, Smithville, IN; and Google Campus,
Mountain View, CA.
See also Fairy Rings at