Why "it’s turtles" not "they are turtles"
It is a third person singular and is used to refer to a thing. If that’s the case, then why do we say:
A well-known scientist (some say it was Bertrand Russell) once gave a public lecture on astronomy. He described how the earth orbits around the sun and how the sun, in turn, orbits around the center of a vast collection of stars called our galaxy. At the end of the lecture, a little old lady at the back of the room got up and said: “What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise.” The scientist gave a superior smile before replying, “What is the tortoise standing on.” “You’re very clever, young man, very clever,” said the old lady. “But it’s turtles all the way down!”
—from A Brief History of Time by Stephen Hawking
I would use it instead of they because the former sounds natural to me, but I wouldn’t be able to explain the convoluted details (unless they are simple) behind it.
This particular it is a Dummy Subject pronoun, Distance it; the construction requires a locative of some sort and estimates the extent of some stretch of (perhaps metaphorical) landscape.
- It's 31 miles as the crow flies from Bellingham to Mt. Baker.
- It's a long way to Tipperary.
- It's just corn out there, as far as the eye can see.
In the quoted sentence (quite a famous one in linguistics, because Haj Ross used it as the Fragestellung (pp iv-v) of his extremely influential dissertation, though he blames it on William James, not Bertrand Russell)
- It's turtles all the way down.
it doesn't refer to turtles, but rather — if it can be said to have any reference — to the "landscape" metaphorized by the old lady's comments (interestingly, it's always a little old lady in the story, though the prominent intellectual varies).
That is, we are seeing, through her imagination, turtles stacked one atop another below us, for as far as we can see (using see metaphorically, of course, but that's normal for imaginary landscapes).
The antecedent of “it” is “what the tortoise is standing on”. The sentence is equivalent to:
You’re very clever, young man, very clever, but what the tortoise is standing on is turtles, all the way down.
It's here is rather like there are. It doesn't have an antecedent, but acts as a subject where there otherwise wouldn't be one.