Crop circles first gained notoriety in the late 1980's in the UK, but weren't actually studied until the 1990's. They have appeared all over the world, most recently in Indonesia. Some are man-made, often for advertising purposes, but many do not appear to be man-made because of the intricacy and the time-frame in which they appeared, literally minutes in some cases. Also a great many of them cannot be understood until viewed from the air.
I said in a previous post that I would pass on information as I learned more about this phenomenon. Alan Foster in his talk "Discloser, Exopolitics, and Planetary Changes" at the Leeds Exopolitics Expo in London last summer gave his interpretation about what they might mean in an hour and a half talk. It appears as though we are being communicated with in such a way so that it cannot be hidden from us.
Most extraordinary is the complex mathematical equations. Some are Mayan translations pointing to planetary time frames, and some appear to be trying to bridge the different religions. Several show the orbits of the planets around the sun and are stunning in their complexity. One seems to be actually pointing to the date of December 21, 2012. Another, with a Mayan headdress may be pointing to the Mayan Prophecies. One appears to show another planet entering into our planetary orbit.
He makes a good case as to why he thinks they are extraterrestrial communications of planetary changes, and why he feels our government has known about these beings for many years. If you are interested you can listen here for free on CMN, or You-Tube here.
I think crop circles are one of those mysteries where you need to make up your own mind about what they might be trying to communicate.