~Fairy Ring

A fairy ring, also known as fairy circle,
elf circle, elf ring, or pixie ring, is a naturally
occurring ring or arc of mushrooms.
The rings may grow to over 10 metres (33 ft)
in diameter, and they become stable over time
as the fungus grows and seeks food underground.
They are found mainly in forested areas,
but also appear in grasslands or rangelands.
Fairy rings are detectable by sporocarps in rings
or arcs, as well as by a necrotic zone (dead grass),
or a ring of dark green grass.
If these manifestations are visible a fairy fungus
mycelium is likely to be present in the ring
or arc underneath.

Fairy rings also occupy a prominent place in
European folklore as the location of gateways into
elfin kingdoms, or places where elves gather and dance. According to the folklore, a fairy ring appears
when a fairy, pixie, or elf appears.
It will disappear without trace in less than five days,
but if an observer waits for the elf to return to
the ring, he may be able to capture it.
~Source~