Can you see the fairy ring of fungi? They are called fairy rings as people believed the fungi grew where the fairies had danced.
Fungi have an underground network of filaments called mycelium similar to roots. The toadstool or mushroom is just a fruiting body which pops up generally after rain and releases spores before dying back.
A fairy ring is created by mycelium growing outwards in a circle, this uses up nutrients in the soil starving the grass and causes the grass to go brown and a mushroom is produced on the outer edge of the mycelium. Once the mycelium in the centre has used up all of the nutrients it dies and puts the nutrients back in the soil and the grass grows back.
As long as they don't reach a barrier they can grow upto 8inches outward per year! One ring formed in France by the fungus Clitocybe geotropa is almost a half mile (600 m) in diameter. This ring is thought to be 700 years old.