poliphilo | Fairy Ring

We have a fairy ring in our front garden. It reappears every year- a circlet of darker, lusher grass with fungi in it.

Question 1: How long do these things last; are they eternal?

Question 2: Are they what inspired our ancestors to built henges and stone circles?

P.S. I apologise to the People of the Hills for using the "f" word. I know they don't like it.