"fairy rings"

I realy do not know but I would haserd a gess that because they spred through their roots and nature being governed by math's I leave it with you.

Certain species of fungus grow outwards in ever-increasing circles. The mushroom is the fruiting body of the fungus, like an apple on a tree. The 'plant' (they are in fact closer to animals) is comprised of threads called mycelium that spread out underneath the soil. They are facinating and beautiful things and I hope you will look after yours!

nicely put Sid

Yes indeed - beautifully described Sid and they are lovely and vey well worth leaving alone....unless they appear on a bowling green ....where they might not be so welcome !! They are virtually impossible to erradicate - and who would want to ....they are enchanting. They can "grow" to 20-30 feet and sometime with "inner circles " developing too.

maybe you like no that the circel is made by lunar gravity agitat the mycelium be so delicat to be afect. cliffo say math-this true, as inner circl are proporton to dimaeter of the secondry circel, rarther like fundamentle harmonic in music. it is usal for the largist circel you see,isnot the largist circl theer is. also the circls grow in not out-the mycelium rech its maxima befor mushroom apeer