I've never heard this "magnetism" claim from anyone but the UFO people, and I don't really trust them to be able to use any instrument that could detect this properly. If you have some trustworthy study that shows proof of anomalous magnetism around crop circles, I'll take it seriously, but until then I'm just going to dismiss that.

And http://en.wikipedia.org/wiki/Crop_circles#Hoaxed_circles is pretty much what I've been saying. Just because some people make sloppy crop circles doesn't mean the others are done by supernatural forces. It might just mean they're done by the more skilled people.