fairy ring
n.— «Everyone who is accustomed to the country knows a “fairy ring” when he sees it. Each ring is only a belt of grass of a much darker green than that surrounding it.…If the grass of these “fairy rings” be examined in the Spring and early Summer, it will be found to conceal a number of agarics or “toadstools.”» —“Fairy Rings” New York Times Mar. 3, 1872. (source: Double-Tongued Dictionary)