Actually, this one seems very likely to be manmade.
I have been formulating a hypotheses on how these are laid out, but haven't really nailed it down yet. I have noticed that a vast majority of these circles are in fields with pathways for farm equipment gridded across them. These could be quite useful.
Have any patterns been found where the grass is laid down all in one direction regardless of the pattern? Or do the directions of flattening always move along the lines of the pattern, or in a circular direction?
