A fairy ring is a circular pattern that is produced by fungi growing in grassland. As the fungus grows out from a central point it forms a circle, and at the edges of this circle the grass changes colour. Meanwhile the original fungus dies off, so all that is left is the expanding ring of fungus growing beneath the surface. Sometimes a ring of mushrooms also appears. These fairy rings keep growing for many years, perhaps even for centuries. They can reach a very large size unless they are disturbed, for example when the land is ploughed.
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