rabbit hole

자료: http://en.wikipedia.org/wiki/Rabbit_hole

A rabbit hole is the entrance to a rabbit's burrow or warren.

In Lewis Carroll's Alice's Adventures in Wonderland, Alice follows a mysterious white rabbit into a rabbit hole to enter 'Wonderland', an absurd and improbable world inhabited by many strange characters. This usage has helped make the phrase refer more generally to any portal into a different or strange world, such as:

• The character Morpheus in the movie The Matrix uses this metaphor when he offers the character Neo the opportunity to enter "the real world".

• The term is used in alternate reality games to describe the initial page or clue that brings the player into the fictional world of the game.
  

• ......

By extension the term has also come to signify any event which triggers a completely unexpected situation.