No, you're not looking at a secret level in a Super Mario game. This is a picture of a medieval illuminated manuscript with a touch of photoshoppery applied. Carl Pyrdum, a medieval scholar, uses our friendly plumber to explain the artistic norms of the medium:
In order to keep the man and his goat in the middle from falling right on through the bottom of the page, the artist draws in little patches of ground beneath them.** Mario, no stranger to platforms that hang in the air as if bolted to the background, would feel right at home with this arrangement.
Link -via Boing Boing