A fairy ring is a naturally occurring ring or arc of mushrooms, they can be so small that you would hardly notice them, or they can grow up to 600 m in diameter! They are mainly found in forested areas, but there are plenty that appear on grasslands.
Fairy ring formation begins with the fungus being spread by airborne spores that originate from the toadstool’s sporocarpus.
Most people associate fairy rings with the circle of mushrooms seen above ground, but there is far more to them than that, most of the organism is below ground, and it is the underlying mycelium that can cause the grass to wither and die through lack of nutrients.
The mycelium consists of a mass of branching, thread-like hyphae, which the fungus absorbs nutrients through, by secreting enzymes to break down the food source and then absorbs the broken down polymers.
An image from the Rainbow Mountains region in Iceland shows evidence of circular plant dieback, from the fungus using up any available nitrogen
There is a lot of superstition surrounding the fairy ring, folklore tells that the fairy rings are made by fairies dancing round in a continuous circle, using the toadstools as tables and chairs for when their legs become tired. Many myths describe how upon entering a fairy ring you become invisible to the outside world and are then made to dance around until you die of exhaustion, which sounds like a particularly horrible way to go!
The Dutch believe that the empty centre is there because the devil puts his milk-churn there, whereas Welsh folklore describes fairy rings as places of fertility and fortune, claiming that crops sown around them and livestock feeding near them flourish and do better than others further afield.