there's a curious mass-equivalence problem implicit in Undertale, at least as I interpret its text: any amount of matter can enter, but only living beings ("with a powerful SOUL", and apparently an ordinary human SOUL isn't powerful enough) can exit. so...what happens to everything? water cascades into the Underground, carrying human refuse with it, but where does the water go? down into an infinite abyss, is the implication.
I've written before about how the "Dark Fountains" of Deltarune have the feeling of singular points, discontinuities in the fabric of the world, where the usual laws of physics break down. The Underground itself has that feeling. Things go in, they don't come out. That's also what many humans say about death—from whose bourn no traveller returns, someone once wrote—although at least there's the chance of repeating things in place. Perhaps Death itself can be regarded as a singular point in the experience of life, a discontinuity or break where it's not possible to predict one's future trajectory based on one's past...a place where your life is, in a sense, non-differentiable.
I've always liked the "Underground = Hades" analogy, but perhaps I need to think more about it.
~Χαρά
