Interesting!

I had just taken the lid off a plastic container which had been in the freezer for a couple of weeks when I noticed the pattern of ice on the underside of the lid. The contents of the container must have been still warm I think and the film of steam froze. What beautiful fern-like patterns!

Here I have fiddled about in Photoshop to produce this fossil fern like image. The faint circles are just the pattern of the plastic lid.

Here is another part of the patterned lid. I wonder how easy it would be to purposefully create icy patterns on plastic in the freezer. Worth a try perhaps.