Rabbit Hole

From ILB

In Lewis Carroll's classic, Alice's Adventures in Wonderland (http://www.literature.org/authors/carroll-lewis/alices-adventures-in-wonderland/), Alice begins her adventure by following the White Rabbit down a rabbit hole, which leads her to Wonderland. The ARG community has borrowed the term to refer to the initial site, page, or clue that brings someone into the game.

Definition lifted from unfiction (http://www.unfiction.com/glossary/).