Fairy ring is a naturally occurring circular ring of mushrooms. The rings may grow to over 10 meters in diameter. They are found mainly in forested areas, but also appear in grasslands. A fairy ring starts when the mycelium of a mushroom falls in a favorable spot and sends out a subterranean network of fine, tubular threads which is known as hyphae. Fairy rings are caused by fungi of the class Basidiomycetes. These fungi can cause the development of rings or arcs of deep green grass as well as dead grass.