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.