Isn't it bigger than any of the cities? It sure felt bigger.
As for this idea: it doesn't exist, but that doesn't mean it can't exist. You could create a non-hostile faction of Falmer, and just have the rest of the NPCs (the ones you talk to) be slaves / thralls / whatever. Give them all either blind eyes, or the all-black eyes. Use the dwemer tileset, as well as the standard cave tileset, with some Falmer set pieces. As long as it's treated as a secret, the lack of lore mentions shouldn't be able to stop you from making this.
Yea, it is massive in relation to other stuff, i was just wondering if there was anything equal to what bg2408 said really "However the Blackreach is nothing like a second continent under earth's surface, "
But yea, even if it doesnt exist, i will still make it,or try to. Basic idea atm is just a underground tunnel network for you to explore, with a small war between the Falmer and the other underground residents, due to falmer having moved down deeper. Though i realise the falmer have been down below for a long time already. Partly inspired by gears of war Lambent vs Locust
Another possibility - although very likely destroyed at this point in history, are some caverns in the Red Mountain region. If I recall correctly there was a sixth house outpost near Vvardenfell's north coast having cavern going way south, having an entrance somewhere behind the ghost fence in Dagoth Ur's territory. However likely collapsed as Morrowind went kaboom.
And then there's the root system in the Shivering Isles, which - according to SI's books - stretches through the whole isles. However only small isolated parts of it were displayed in game (important parts nonetheless).
Some Ayleid structures in Cyrodiil were also pretty big. Best bet for Skyrim however I think is the Blackreach. Everything else is just... somewhere else. However the Blackreach is nothing like a second continent under earth's surface, more some really expansive caverns.
Thanks. Will see if i can find some reading on these things.
If Blackreach doesn't already fit the definition of a massive underground cavern, I don't know what does...especially if you include all the gigantic dwemer ruins that are attached to it.
Other than that, no. There is not an 'Underdark' in TES, at least not that we know of for sure.
Only other places I can think of were some caverns in Morrowind, namely the caverns within Dagoth Ur itself, and maybe the caves under Kogoruhn that connected it to Red Mountain.
Or the ruins of Old Mournhold in Tribunal, those were huge and connected to the (gigantic) sewers and all those weird winding caves with the daedric ruins inside and also those huge dwemer ruins. I think all those were connected. if anything, those caves would count probably more than Blackreach I think.
Again, Thanks.
Thats pretty much all i needed to know. I did doubt whether there was many things like what i was thinking of in Tes, but never hurts to ask.
Was mainly just for basic ideas anyway, as no matter what existed already, Whatever i make wouldnt fit into lore much, if at all.
Might be biting off more than i can chew with this idea anyway. I realise now after modding oblivion that you want to start small, but ive got a bit more experience now and i might just plod away at it for a while
So, again, Thanks for replys.