Symbolism, Imagery, Allegory
In a twist that Godbole would surely appreciate, nothing in the novel is actually something. That is, it's a symbol. A symbol of – nothing.
The novel begins with the word "nothing" in its first sentence. You might have noticed that the novel seems obsessed with gaps and holes. The novel is roughly structured like a donut, with a big hole where Adela's experience in the cave should be. But if you think about it, even though nothing is written about Adela's experience in the cave, it doesn't mean that nothing happened or that nothing can be said. In fact, it's probably the most interesting part of the book precisely because it's missing. As the narrator comments on the Marabar Caves – which are just a big series of holes – they are "extraordinary." The extraordinariness (if that's even a word) of nothing – one of the more mysterious and certainly compelling motifs in the novel.