I was fairly sure I knew where the expression “down the rabbit hole” comes from – from Alice’s Adventures in Wonderland, by Lewis Carol, and her entry into Wonderland by following a white rabbit and tumbling down his hole. But I was curious how it became a metaphor for any entry into the unknown, the disorienting or the crazy-making.
The slang expression arose in the drug culture for a psychedelic experience. Although I didn’t find a citation for its first use, the phrase has spread.
We mean that we got interested in something to the point of distraction—usually by accident, and usually to a degree that the subject in question might not seem to merit. newyorker.com
The New Yorker’s writer goes on to attribute the current popularity of the phrase to the internet, and explains why it perfectly captures what happens to us as we surf. Fun article.
Stackexchange.com warns against the mistake of diving for a rat hole when you meant a rabbit hole. A rat hole is a North American phrase for a waste of money or resources, which are commonly “poured” down the rat hole in a short-sighted move. That’s a bad trip.