I'm not a great one for illusions, possibly because a couple of times when I have been concentrating on them they have turned into screamers, those horrible ones where the picture suddenly turns into a horrific image, such as a close up of the girl in The Exorcist, with extremely loud frightening screams. I hate those things.
Having said that, I came across this illusion again recently. The first I saw it was many years ago where someone had an enlarged one on their front lawn as part of a Christmas lights display. The instructions were to look, then look at the clouds. It was eerie.
If you haven't seen this before, spend 30 seconds looking at the 4 dots in the centre. Then quickly stare at a blank surface, such as a wall or a ceiling. It takes a few seconds but you should see an image appear.