"Turtles all the way down" is a jocular expression of the infinite regress problem in cosmology posed by the "unmoved mover" paradox. The phrase was popularized by Stephen Hawking in 1988. The "turtle" metaphor in the anecdote represents a popular notion of a "primitive cosmological myth", namely the flat earth supported on the back of a World Turtle.

A comparable metaphor describing the circular cause and consequence for the same problem is the "chicken and egg problem". Another metaphor addressing the problem of infinite regression, albeit not in a cosmological context, is Quis custodiet ipsos custodes? The same problem in epistemology is known as the Münchhausen Trilemma.