Not crop circles, mind you, but variations in crops that are indicative of subsurface archaeological features. A heat wave and partial drought in Great Britain have rendered such marks unusually prominent.
Last week the Royal Commission on the Ancient and Historical Monuments of Wales posted a well-illustrated article (schematic images embedded), showing how ancient earthworks create alterations in crop size and color by allowing water to be retained during times of scarcity. The advent of drone photography has obviously simplified the detection process immensely.
I recommend visiting their link to see the awesome gallery of British cropmarks, but today I'll embed a different image I found today at Wired:
A previously unknown henge has been revealed in Boyne Valley, in the Brú na Bóinne UNESCO world heritage site, in Ireland's County Meath. Stretching 200m in diameter, 750m from the famous Newgrange monument... The henge is thought to date from the late Neolithic period, up to possibly the Bronze Age, from about 3,000 BCE...Wow. I think I'll go climb a ladder in my front yard...
The henge would have been made out of timber with two concentric circles, which would possibly have been 'linteled' with horizontal supports as well. "This is a time period where they're building particularly in timber and earth, as opposed to stone which went before," Davis says."We have this bizarre broken ditch, which we don't really necessarily understand yet and that's the most unusual thing about it," Davis says. This ditch is causewayed, broken into lots of little bits, forming a "permeable boundary" meaning it's not a form of defense. Although there are discernible entries and exits, you could in theory enter the structure at any point. "It makes it much more like a symbolic enclosure, rather than a real enclosure."This all points to the idea that the structure was used for ritual ceremonies that involved feasting, gathering and trading together.