Metaphor for the conceptual path which is thought to lead to the true nature of reality. Infinitesimally deep and complex, venturing too far down is probably not that great of an idea.
Taking hallucinogenic drugs can be considered "tripping" down the rabbit hole, but it is also explored through philosophical and existential thinking.
The origin of the term is from the rabbit hole in Alice in Wonderland which leads into Wonderland.
You take the red pill and you stay in Wonderland and I show you how deep the rabbit hole goes.
- Morpheus, from The Matrix (I couldnt think of anything original, sorry)