What is the source of the allusion in the sentence below?
Will's research on nineteenth-century philosophers led him down the rabbit hole.

 - literature
 - the Bible
The source of the allusion down the rabbit hole is literature.
Lewis Carroll's Alice's Adventures in Wonderland tells the story of a young girl who follows a white rabbit down a rabbit hole and finds herself in a series of adventures in a surreal world.
The allusion down the rabbit hole means on a strange or difficult exploration.