Dark Green Grass Circles in Lawn - WHY?

I have a very strange problem regarding my lawn that has perplexed me and my wife as well as my neighbors. Specifically, in recent weeks, our lawn has developed bright green circles of grass amidst the lesser green grass. From a distance, it appears that our lawn has the 'mumps.' The greener grass in the circles is higher and evidently is growing faster than the less green grass that surrounds all of these circles. The dark green circles of grass, and there are many, many of them, are all approximately a foot+- in diameter and are spaced anywhere from one foot to two feet from another circle or circles.
The strange things:
These dark green grass circles almost stop where both of my neighbor's lawns begin on both property lines (no fences).
All lawns on our block are maintained and cut by the same lawn service. We do have a female dog but she rarely, if ever, goes on our lawn. She usually does her business in other places when I walk her. There are just two many green grass circles to have one dog responsible for this. None of my immediate neighbors have a dog.
I have not given our lawn any fertilizer or other treatments. Our lawn service does this.
Does anyone have an idea what these dark green grass circles may be and what may be causing them? Are my wife and I living in a vortex?
Any thoughts or answers will be appreciated...........
Tx,
Mark Z.
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Aliens ?

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There was a tree there years ago.They call it angle circles I think.
From Mel & Donnie in Bluebird Valley
http://community.webtv.net/MelKelly/TheKids
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Do you also have toad stools in those areas, and does it look anything like this? (fairy ring)
http://tinyurl.com/34vanv
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