I saw an image on Pinterest by Witchoria, of a landscape photo, which had a circle mask, contain the same image inside itself, upside down. It made me look at it like a portal, so I wanted to have a play myself! So I got 2 of my medium format film photos which had similar horizon lines and then mucked about with them - adding a lil' lens flare and shadow to give it a bit of realism. It's good fun!