I've seen a few former crop circle makers from the UK show how they made them, which was bassically with a board, some rope, and a design they pre drew to scale of the field they owned. Walking on the boards bent them over evenly without any footsteps, and they walked along the paths you see in the image above this post.
It was on TLC a few years ago, but I imagine you can YouTube it with some search like "how to make crop circles" or something like that.
But basically, no, it's not aliens.... just some lonely farmers with a lot of time on their hands..... and besides I imagine aliens have more important things to do then flatten our food with pretty pictures.