Don't fall in that huge crater! ... made out of paper.
To be honest, I'm not sure what it's made out of, but it's some sort of thin material. I guess the artist, Erik Johansson, printed a realistic looking "hole" onto some "paper" and set it up in a public area.
This optical illusion is best viewed from a specific angle, since people walking on it will just see it as... a large artwork on the ground.
I just thought this was pretty neat; I've always loved optical illusions but this takes the cake!