Fairy ring?

This weekend, StfRon and I headed to Bishop Hill for some shopping and lunch, and some geocaching in the area.
Some of my ancestors settled in the area, so we stopped at the La Fayette cemetery on the way back to see if any familiar names were to be found. I did find a handful, so I will have to see if I can fit them into my family tree.
On a strange note, we noticed two rather large almost perfect circles of mushrooms growing in the grass in the cemetery. The grass where the mushrooms sprouted was darker green than the surrounding grass.
StfRon said this is known as a fairy ring. I had never heard of such a thing. I snapped a picture of one of the rings, but I wish there had been something to stand on so I could have had a better vantage point.