This is the Discworld, which travels through space on the back of four elephants which themselves stand on the shell of Great A'Tuin, the sky turtle.

Once upon a time such a universe was considered unusual and, possibly, impossible.

But then... it used to be so simple, once upon a time.

Because the universe was full of ignorance all around and the scientist panned through it like a prospector crouched over a mountain stream, looking for the gold of knowledge among the gravel of unreason, the sand of uncertainty and the little whiskery eight-legged swimming things of superstition.

Occasionally he would straighten up and say things like ‘Hurrah, I've discovered Boyle's Third Law." And everyone knew where they stood. But the trouble was that ignorance became more interesting, especially big fascinating ignorance about huge and important things like matter and creation, and people stopped patiently building their little houses of rational sticks in the chaos of the universe and started getting interested in the chaos itself - partly because it was a lot easier to be an expert on chaos, but mostly because it made really good patterns that you could put on a t-shirt.

And instead of getting on with proper science scientists suddenly went around saying how impossible it was to know anything, and that there wasn't really anything you could call reality to know anything about, and how all this was tremendously exciting, and incidentally did you know there were possibly all these little universes all over the place but no-one can see them because they are all curved in on themselves? Incidentally, don't you think this is a rather good t-shirt?

Compared to all this, a large turtle with a world on its back is practically mundane. At least it doesn't pretend it doesn't exist, and no-one on the Discworld ever tried to prove it didn't exist in case they turned out to be right and found themselves suddenly floating in empty space. This is because the Discworld exists right on the edge of reality. The least little things can break through to the other side. So, on the Discworld, people take things seriously.