rabbit hole

Short definition from Wiktionary

Noun

1. 1
   A bizarre world, where everyday rules do not apply.
2. 2
   A way into such a world.
3. 3
   A time-consuming tangent or detour, often one from which it is difficult to extricate oneself.
4. 4
   The initial clue that leads to an alternate reality game.
5. 5
   Used other than figuratively or idiomatically: see rabbit, hole. The entrance to a rabbit warren or burrow; the whole warren or burrow.
6. 6
   Alternative form of rabbit hole.