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".
In the film What the bleep do we know!?
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.
Rabbit Hole is the name of a play by David Lindsay-Abaire.
Rabbit Hole Ensemble is a theatre company in New York City.
Rabbit hole is the common theme of the introductions to Armin van Buuren's A State of Trance Yearmix. Every Yearmix is compared to a journey through the rabbit hole. In Yearmix 2008, the rabbit hole is described to be "boundless in the extremities of time, immeasurable in its capacity, perpetual in its own right."