X0 is turtles, as recorded in Stephen Hawking’s 1988 A Brief History of Time:
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!
Whenever this story originally appeared, it does seem to have become popular as a result of Hawking’s retelling:
X must be something that can be pluralized: actors, cans, features, caches, big bangs, cronies, and fraudsters all occur in the variable position. It often seems that if a person suggests that it’s “caches all the way down“, he means that many things in a system may be called something else but ultimately can be modeled as caches. Whether we think of gravity and centrifugal force rather than the balancing of a giant turtle as what holds us on the earth, it doesn’t make much difference in most of our day-to-day actions. I’ve also often heard “it’s X all the way down” as the answer to “but then what? but then what?” type questions–at some point the last thing you understand in a sequence of events is what goes all the way down.
The Snowclones Database 