The Biggest…..

…The Biggest Fairy Ring I have seen.

I think this is Clouded agaric sometimes known as Cloud funnel.
Clitocybe nebularis.

These rings are found in Europe and North America. They have in the past been the subject to much Myth and Folklore linked with witches and the devil.
This ring is about 20 metres in diameter. It was difficult to photograph the whole ring without having a drone.

Some even larger rings in the south of England are thought to be hundreds of years old.
If you want to know how they do it or why they do it I recommend the article on Fairy Rings in Wikipedia.