This unusual phenomenon in the mycology world is called a "fairy ring". People finally can explain them & why these odd rings of toadstools & mushrooms form, but once in a rare while a lawn will have one or more appear. Some folks theorize that a tree or some other organic object once inhabited or was buried in this spot & the mushrooms have grown in a similar shape to the object. Is it true? Maybe, maybe not? Makes some connective sense since most fungi grow from a tangle of filaments that, in mass are called Mycelium and the Mycelium spreads in the nutrient source in the ground which ~could~ have been a rotten out stump or pole ring. This one is about the diameter of a bushel basket.