A fairy ring in a neighbor's lawn. Fairy rings are underground all the time, because mushrooms/toadstools are reproductive structures coming from the main organism, which is fibers (mycelia) underground. These mycelia gradually spread out from the center. All these mushrooms are from the same species, and, in a sense, they are all from a single organism.
For more on fairy rings, see the Wikipedia: en.wikipedia.org/wiki/Fairy_ring
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Thanks for looking. The Wikipedia says that there are about 60 species that do this. I have no idea which one this is. If you can identify it, I'd appreciate it. Cornell Fungi (see comment below) has tentatively identified this as Amanita jacksonii.