I'm actuall pretty sure that humans made all of those.
The ones that are apparently genuine usually entail circles, generally many circles criss-crossing each other, and cut into the crops at different heighths. From what I understand, they only take a couple of seconds, because they're using holographic projections.
The aliens aren't coming here hoping we'll turn them into deities. They genuinely want to give us a message. These are spiritual messages. It's believed that many of them are operating on higher dimensions, so I doubt they'd be so vain as to go through a wormhole in another galaxy, or possibly different dimension just to come here and inscribe pictures of themselves.
