"down the rabbit hole", a metaphor for adventure into the unknown, from its use in Alice's Adventures in Wonderland a slang expression for a psychedelic experience, from the same usage http://en.wikipedia.org/wiki/Rabbit_hole It comes fro the story "Alice in Wonderland."

Have you ever read "Alice in Wonderland?" Well, she followed a rabbit down the rabbit hole and found herself in a bizarre world. So the expression means to find oneself in a bizarre, strange situation.

The expression "down the rabbit hole" is a metaphor for adventure into the unknown, from its use in the book "Alice's Adventures in Wonderland". It can also mean an initial page or clue that brings the player of an alternate reality video game into its fictional world.