Wired went out and found Temporary Temples, where (in addition to a decade’s worth of crop circles) there are pictures of white horses, monoliths and other oddiments of the English countryside.
I found the horses especially fascinating. They are chalk figures which are (according to Wikireality) usually created by the cutting away of the top layer of relatively poor soil on suitable hillsides. This exposes the white chalk beneath which contrasts well with the short green hill grass and the image is clearly visible for a considerable distance. Here’s a link to some more info about the Uffington White Horse, wiltshirewhitehorses.org.uk and the Hillfigure Homepage.