Big dark green circles in your lawn are often called 'fairy rings', but they are actually a soil inhabiting fungi from the Basidomycetes class. The fungi causing the big green circles can be from a wide variety of species within the Basidomycetes class, but the result is the same no matter which fungi is at work. Fairy rings occur when the fungi begins living on organic matter that's decaying in the soil. For example, if you removed a tree, the remaining roots will start to decay. Fairy rings from fungi may also occur where the grass is too thick and some of the roots are dying due to lack of available nutrients. You may have mushrooms sprouting or rings of dead grass inside the fairy rings. The pattern and shape of the big dark green circles can change from season to season as the fungus continues to move onto other decaying matter.
The dark green shade of the circle comes from the fungi breaking down the decaying organic matter, which can produce high levels of nitrogen. The increased nitrogen fertilizes the area of lawn and creates a dark green formation. The mycelium of the fungi seeps deep within the affected soil and becomes so thick that it prohibits moisture from penetrating the soil. This is the reason you may see dead grass inside the dark green circle. Some fairy rings may only be 2 inches in diameter, while others can grow to 50 feet in diameter.
Treatment strategies for big dark green circles in your lawn can include applying fertilizer to the surrounding area in an effort to equalize the nitrogen levels. This may balance the green contrast on your lawn. You could also aerate and irrigate the area with large quantities of water to help increase the rate of decomposition, thereby minimizing the lifespan of the fungi. Another possible treatment strategy is to dig up the fairy ring and reestablish it by planting new seed. This will not guarantee that the fungi will disappear; decaying matter well beneath the soil may still be providing a feast.
You could also try to kill the infected area with herbicide and then till the soil in several directions to spread the mycelium evenly over the surrounding area. The types of fungi that cause big dark green circles on your lawn are antagonistic towards each other. If the mycelium is intermixed, new fairy rings will not arise.