I was re-reading The Silver Chair recently, the 6th book in the Narnia series. Towards the end of the novel,
when they've freed Rilian,
he says,
"he had always reached these outlets by going in a ship across the Sunless Sea" (ch. 13)
Furthermore, the gnomes and the "stately figures taller than men" bear a resemblance to D&D's deep gnomes and drow. Forgotten Realms seems to have the varying levels as well.
Is there any direct evidence (i.e. quotes from designers or hidden Narnian references) implying that the Underdark was inspired by the world in The Silver Chair?
