What are fairy rings?

An EarthSky Facebook friend wrote: My girlfriend and I saw this big circle of mushrooms. She said ‘Wow a fairy ring!’ I’ve actually noticed this before – mushrooms growing in big circular patterns. What causes it? Fairy ring of Clitocybe nebularis (“Clouded Agaric”) Photo credit: Wikimedia Commons That circular pattern of mushrooms is known as a “fairy ring” from the old belief that fairies danced inside the circle of mushrooms.