A number of mushroom species grow in distinct circles that have been nicknamed fairy rings. The name is a reference to tales of folklore suggesting that fairies would dance within these areas, but the actual biological explanation is much less fanciful.
Fairy rings start as a single mushroom spore, which begins growing by putting out an underground root network called mycelium. Each summer it produces fruiting bodies - mushrooms - which are temporary reproductive bits akin to flower blossoms. The mycelium draws heavily on the nitrogen in the soil as it grows and fruits, and the mushrooms appear at the outer edge of the network, where the nitrogen is richest. As the mycelium network expands, so does the fairy ring formed by the mushrooms. There are two types of fairy rings. Tethered rings are formed by species that are partially dependent on the roots of certain tree species for nutrition (commensalism), and often occur with a tree growing at their center. Untethered rings don't require tree roots and so are often found in meadows and lawns.
The largest fairy known fairy ring is in France; it is about 2000 ft (600 m) in diameter and estimated at 700 years old. Most fairy rings don't achieve such sizes; typically, rings are less than 33 ft (10 m) across.
Shared from NC Extension Master Gardeners: https://www.facebook.com/NcExtensionMasterGardeners