DIMITRI: If Atlas holds up the world, what holds up Atlas?
TASSO: Atlas stands on the back of turtle.
DIMITRI: But what does the turtle stand on?
TASSO: Another turtle.
DIMITRI: And what does that turtle stand on?
TASSO: My dear Dimitri, it's turtles all the way down!
(from Plato and a Platypus Walk into a Bar.. Understanding Philosophy Through Jokes by Thomas Cathcart and Daniel Klein, page 1: an Introduction)