Fairy Ring

"The symptoms of fairy ring appear in patches, rings, or arcs that are initially 1 foot or less in diameter, but expand in size year after year, reaching up to several hundred feet in diameter in old turf stands. Most fairy ring fungi do not infect or parasitize the turf. Instead, growth of these fungi in the soil can indirectly affect, or even kill, the turfgrass above. "
This passage comes from NC States Turf DIsease blog.
Fairy ring looks like this.