Crop circles

These are a surefire warning of the presence of elves.

A rash of them happened one summer in Lancre. In the Royal bean crop and flowerbeds, in Pewsey Ogg's watercress, that he was proudly showing off to his Nanny at the time, and in Granny Weatherwax's herb beds.

By magical resonance, Unseen University was also affected: Mustrum Ridcully got a particularly visible one on his scalp, leading to much hilarity among Faculty members, Ridcully's indignant harrumphing that he was NOT going bald, and his consequent professional interest in spells, nostrums and devices aimed at restoring the growth.