I'm gonna go out on a limb now, a brittle, precarious limb, and try and start a discussion, (which I kinda already did once already which ended poorly..) about the Realities behind Nirn, the Mundus, Oblivion, Aetherius, and the greater cosmos this Universe inhabits. Let me put my tinfoil hat on.
The Basics of what we know. Nirn is a life sustaining world, similar to a planet. It's nearly identical to our Earth, land, oceans, a sky, breathable air, weather, climates etc. Nirn (perceived from the Mortal eyes on the ground) has two moons that appear to orbit Nirn, Masser and Secunda. A Planet with Moons, okay. The night sky has stars in all directions, and the day sky has the sun, Magnus, which Nirn appears to be orbiting. So, to recap, and to describe it in our real world terms, Nirn is a habitable planet with two moons orbiting it's parent star Magnus. Now onto the more fantastical parts.
The realms of Oblivion are separate from the Mundus, you can't get to one by conventional means, ie. you can't just walk there. You can however get there by means of Portal and other forms of "Rifts". What the Canon explains is these realms are planes of existence, both there yet not there, all forms of fancy mind boggling stuff. But, my personal theory regarding the Realms of Oblivion. They're not planes separate from Nirn, but they ARE, Nirn. The Realms of Oblivion are Dimensions, different versions of Nirn itself. Inter-Dimensional travel is a common SciFi trope, but it's still the least bit plausible to explain, say, why there are intelligent humanoid creatures (The Dremora, Xivkyn, Xivilai, Dark Seducers, Golden Saints). These are the Races that inhabit the other "Nirns" instead of the Races we know of.
And then there's the Divines. The Aedra, the Daedra. We've been told of this "Cosmic Game" that's being played, this grand scheme between the Princes and that Nirn is firmly within the cross-hairs. What these Princes, the Divines, were just creatures from other Dimensions that literally were just playing a game with us? One giant charade. I want to quote something from Men in Black. "A person is smart, people are dumb, panicky, dangerous Animals.". The Mortals of Nirn are a Medieval people who's greatest common technological achievement are the levers that open doors somewhere in a Dwemer Ruin. We know that Science is a prevalent force in the Universe, and any kind of Science is considered Magic by people that don't fully understand it. What if these Daedra are using a Science that the Mortals of Nirn just can't even hope to comprehend, so it's written off as more Magic? I'm not trying to write of Magic as a whole however, there can definitely be forces of Nature that isn't available in our real world. Even we have discovered Humanity isn't making full use of our minds, and the Human Brain is the most complex, sophisticated computer in nature. The kind of power that could be unlocked could range from Telepathic, Telekinetic, or in Tamriel's case, Magical?
Time to rest from my Tinfoil hat now, I know there's more I think about but I'm crap at writing out well thought opinions, it's why I never got straight A's in English. :P I guess I secretly hope that behind all the Lore we know of Tamriel is in reality much more easy to understand, but then again I'm not the game's writers, Nirn is most likely actually just one big magical place full of magical things and angry magical people from other magical planes that defy every law of physics imaginable, that is what Fantasy is supposed to be about right? Perhaps someone from ZOS should come in here and tell me I'm wrong already. lol
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