Before we knew the world was round, the seemingly logical explanation was that it was flat. And that it was supported by the concept that it was balanced on the back of a turtle. The unanswerable cosmological question became, "and what does that turtle stand on?"The only explanation was, "on the back of another turtle, and another turtle, and another turtle ... turtles all the way down." Such a cosmological explanation is known as the explanation of the infinite regress with the problem being there is no end.
The phrase has been used in many ways to explain, or make light of, numerous breakdowns in logic: physics, cosmology, epistemology, psychology, et. al. It has even been used by Supreme Court Justice Antonin Scalia as an explanation of the merits, or lack thereof, of a court decision in 2006.