At last we have the answer. Crop circles are caused by stoned wallabies. You think I'm kidding? Then look:

Wallabies are getting stoned on Tasmania's opium poppy crops and hopping about in circles, authorties say. Attorney-General Lara Giddings says wallabies have created crop circles of squashed poppies as they increasingly hop in to the fields eating the poppy heads. That causes them to get high and run around in turns creating crop circles, she told a budget estimates hearing.
If you don't have any wallabies in your area, then it's probably the stoned sheep:

Tasmanian Alkaloids field operations manager Rick Rockliff said "There have been many stories about sheep that have eaten some of the poppies after harvesting and they all walk around in circles."

Now the real question ... if you take their poo and make it into paper (as has been done already in Tasmania, believe it or not), then use it to roll your smokes ... will it do anything? I know coffee lovers go nuts over some weird Indonesian coffee that's seen the beans go through the digestive tracts of some kind of monkey ... could roo poo pot be the next big thing in the drug world? It's certainly got a catchy name.