I took this yesterday evening ...
A great deal of folklore surrounds fairy rings. Their names in European languages often allude to supernatural origins; they are known as ronds de sorciers ("sorcerers' rings") in France, and Hexenringe ("witches' rings") in German. In German tradition, fairy rings were thought to mark the site of witches' dancing on Walpurgis Night, and Dutch superstition claimed that the circles show where the Devil set his milk churn. In Tyrol, folklore attributed fairy rings to the fiery tails of flying dragons; once a dragon had created such a circle, nothing but toadstools could grow there for seven years. European superstitions routinely warned against entering a fairy ring. ...
Western European, including English, Scandinavian and Celtic, traditions claimed that fairy rings are the result of elves or fairies dancing. Such ideas dated to at least the mediæval period; The Middle English term elferingewort ("elf-ring"), meaning "a ring of daisies caused by elves' dancing" dates to the 12th century. In his History of the Goths (1628), Olaus Magnus makes this connection, saying that fairy rings are burned into the ground by the dancing of elves. British folklorist Thomas Keightley noted that in Scandinavia in the early 20th century, beliefs persisted that fairy rings (elfdans) arose from the dancing of elves. Keightley warned that while entering an elfdans might allow the interloper to see the elves—although this was not guaranteed—it would also put the intruder in thrall to their illusions.
The folklores of the British Isles contain a wealth of fairy lore, including the idea from which fairy rings take their name: the phenomena result from the dancing of fairies. In 19th-century Wales, where the rings are known as cylch y Tylwyth Teg, fairies were almost invariably described as dancing in a group when encountered, and in Scotland and Wales in the late 20th century, stories about fairy rings were still common;some Welsh even claimed to have joined a fairy dance. Victorian folklorists regarded fairies and witches as related, based in part on the idea that both were believed to dance in circles. These revels are particularly associated with moonlit nights, the rings only becoming visible to mortals the following morning. ...
Many folk beliefs generally paint fairy rings as dangerous places, best avoided.... Superstition calls fairy circles sacred and warns against violating them lest the interloper (such as a farmer with a plough) anger the fairies and be cursed.... Even collecting dew from the grass or flowers of a fairy ring can bring bad luck. Destroying a fairy ring is unlucky and fruitless; superstition says it will just grow back.
A traditional Scottish rhyme sums up the danger of such places:
He wha tills the fairies' greenFor better or worse, I saw no faeries.
Nae luck again shall hae :
And he wha spills the fairies' ring
Betide him want and wae.
For weirdless days and weary nights
Are his till his deein' day.
But he wha gaes by the fairy ring,
Nae dule nor pine shall see,
And he wha cleans the fairy ring
An easy death shall dee.