Now don't step into one as you mighty die young, disappear or be forced to dance with the fairies until you drop dead or go mad! Some believe you have to run around the ring 9 times ( but not 10 as that doesn't work!) & under a full moon & in the direction that the sun travels in the day...have you got all that? Well a simpler way may be the belief that if you wear your hat back to front that'll confuse the fairies!
There's still a lot of belief though, that a mushroom (fairy) ring brings a lot of luck. There are some that believe that sheep eating fairy ring grass, will then flourish & that crops grow better around them.
However the simple scientific explanation is that fungi like toadstools & mushrooms are nitrogen hungry & so as they deplete an area the successful spores are those that are further out & so a gradual ring forms becuase in the center there is no food for them! However if the soil where they are successfully growing is nitrogen rich you can see why those crops might be doing so well!
Knowing your mushrooms is a dying art & there are few now that would chance knowing the differrence between the hugely poisonous ones from the safe & nutritous ones. Any way nowadays it is easy to grow your own with mushroom kits & be content that these ones will not kill you - fairies or not!