Based on Earth but beyond the ice wall.

Oh, that's a factor my model doesn't include. It's a good Plato's cave analogy, but I think the cave door is not the same as the perimeter portal. To me the refractive index causes any projected light from beyond the ice wall to have a different path so we'd be able to track it as a separate (searchlight) phenomenon and not just a single projection. Different models have different boundaries and sometimes they're isomorphic and sometimes not. I haven't had the speed and mass be issues for an excited plasma approach, but then my numbers are just spitballed. I'm still going with the wormhole.

I do think it's more mysterious What is beneath us. Hard to imagine infinite ground beneath us. Seems that there would be an end to the floor and perhaps that leads to another space with stars and then another level of another Earth. Maybe there are infinite levels of Earth stars and atmosphere Above us and below us. And maybe laterally land extends for infinity. There's too much speculation to ever draw conclusion.

Well, I was just able to run a calculation that the quantum nature of the universe means the earth cannot be more than 616,000 km deep. However that's only a first approximation and a better model would yield a smaller number: WIP. There is a quantum boundary beyond which action becomes meaningless; the boundary above us seems similar, and as I said the joining of these two manifests in refraction in the air.

However, the other universes would be reached not by drilling or rocketing but by connection in other dimensions, which is much more efficient. So even if they really are connected in Euclidean 3-space they would still be more rightly considered as connected in higher-D.

But why are we all here. Even if we are Just economic slaves or a livestock of some kind. Why would the overlords that control this world be here? Why does spirituality exist at all?

Every lord and slave is accountable each on his own level: lordship and slavery are merely different degrees on a spectrum of liberty, lords have slightly higher freedom. Freewill exists for the purpose that Good be done freely (not robotically), which requires that a certain amount of evil be permitted for a certain period, limited to that which the universe can bear. (Black hole theory actually informs this, because black holes contain bearable entropy, not unbearable.) The purpose of spirituality is then this:

For, brethren, ye have been called unto liberty; only use not liberty for an occasion to the flesh, but by love serve one another. For all the law is fulfilled in one word, even in this; Thou shalt love thy neighbour as thyself.