It feels to me that the proto-Underdark is as much about depression and despair as it is about an extensive subterranean ecosystem. A lot of The Silver Chair is allegorical in a Pilgrim’s Progress lite sort of way - the Gentle Giants are, well, giants (and sort of referencing Gulliver’s morally superior Brobdinagians) but they’re also representing oblivious middle-class adults who caution children that religion and fairy-tales are nonsense.
Underland is therefore a setup for the Lady’s atheist pitch to our heroes, that all Narnia and Aslan and all good things are an illusion, and for Puddleglum’s debate with her (his argument boils down to a desperate ontological one of “well, I want to believe in it whether it’s real or not”). It’s commonly said that this scene reflects Lewis’ own public
debate in 1948 with the philosopher Elizabeth Anscombe, which he’s generally said to have lost and which may have put him off writing any more Christian apologetics. Both Lewis and Anscombe underplayed the importance of their debate. In any case, the themes here foreshadow (ha ha) Lewis’ ultimate pitch in The Last Battle that it is real life that is the illusion (“shadowlands”) and God who is truly real.