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 deathβfrom whose bourn no traveller returns, someone once wroteβalthough 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.
~ΧαΟΞ¬
I've been thinking about this for a fic I've been writing. The somewhat boring conclusion I've been forced to come up with is that any inanimate matter may pass through the barrier. So far this has only been confirmed with fluids (water, magma, and air) because everything would be fucked otherwise.
One idea I'm toying with though, is that while matter may pass through, electromagnetic radiation cannot. Think about the implications. From within the barrier, the sun shines through. But if you're outside the barrier looking in, you only see impenetrable darkness.
Imagine you're hiking on a mountain with a friend. You come across a cave opening. It's dark inside. Darker than it should be. So dark it seems to absorb any light that enters. You try shining a light to see inside, but the beam gets swallowed by the dark. You pick up a small rock and throw it toward the cave. It disappears completely. Your friend, braver than you are, picks up a long stick and approaches the cave. They hold the stick in front of them and as the tip enters the cave, it disappears from view completely. The friend is able to pull the stick back out. It seems undamaged. What if they just try to put their hand through...
It won't come out. Their hand is stuck and it won't come out. They start flailing around in a panic. Every inch of their arm that enters the darkness is subsumed by it. They try to calm down, to stand in place. You grab them and try to pull them out with all your might, but they start yelling at you to stop, it hurts! You stop.
You tell your friend to wait. You'll go get help. You'll find someone who can save them.
You return later with a rescue party. They're gone. Your friend is gone. Did they escape? The only footprints leading away from the cave are yours. There's only one way they could have gone.
They went inside.
They are never heard from again.