Would it be possible to do? A new land mod but entirely underground like a dungeon.

Or would you just have to build an incredibly big dungeon from scratch. I havent much experience with new lands.

I'm sure it could be done. The level of "pain in the ass-ery" would be dictated by its design. I don't see a reason why you couldn't use the exterior lands pieces in an interior location. The only problem would be with the lod,heightmap and using things that are typically associated with an exterior cell. However if you just made it a link to a new worldspace and made the lighting and weather conditions act more like an interior, then use some of the interior pieces to make it so the area has boundaries that are more like an underground location it would work a lot better.

A new worldspace with a sky that looks like a cavern roof sounds entirely plausible. Got any plans?

i dont really have any plans but i had an idea for maybe trying to recreate skyrim but underground so you have windhelm depths, winterhold depths etc etc..

I think that would be very hard and very painstaking to create though. So maybe just a new underground land that is an ancient nordic town/world.

I dont have the skills to pull this off at the moment but its good to know thats its plausible, i might work towards it. If anyone likes the idea and feels they have the skills, feel free to give it a go

...so do you mean a "depths" under each city, like a dungeon? Or the entire worldspace, but underground? Because for the latter, it'd be fairly easy to adapt sky textures, change up flora, and that kind of thing, to fit. Easier than making a new world with those changes.
Or both? Might be viable as duplicates.

edit: by both, i mean having an overworld as normal skyrim, and an underworld as edited skyrim.

There are several approaches you can take:

1. Journey to the Center of the Earth - it's a land with sky, but it's underground
2. Qarl's The Underground mod for Morrowind - create a sewer system leading to various subterranean buildings (c/f Vampire The Masquerade: Bloodlines)
3. Dragon Age - functioning Dwarven cities accessible through mountain tunnels

Obviously 2. would be the most lore-friendly, since Morrowind's Tribunal expansion made major use of the sewer system and still-inhabited ruins, though I'd advise against black PVC if you're going for something that doesn't raise eyebrows with purists.

You could also do 3. and just explain that, like Markarth, a bunch of Nords found some Dwemer ruins and moved in.

I did an underground village once, for Oblivion, by extending an old Ayleid ruin and making up a story about how the Ayleids hadn't really died out at all - like the Falmer, the ones that hadn't been assimilated into the general elven populations had just hidden away undiscovered. The biggest challenge for me was lighting, but in that case I used the magical source of the Varla stones, which capture moonlight. There is a magical light source and glowing mushrooms you can use - just pick an explanation and stick to it - but once you start thinking about things like "where does the light come from to make the plants grow?" the rest sort of falls into place naturally.

it would be the entire skyrim worldspace, with subterranean areas such as markarth depths, windhelm depths.

storywise, i would proberly go with it being some kind of area for the poor or something similar.

in reply to princess stomper, id proberly go with 3 but instead of dwarven cities ive have them as some kind of giant nordic ruin of a underground land sized town created to house the poor and criminals etc because of over-population. Although around markarth, the area could include a dwemer ruin connected in someway to the underground worldspace. Winterhold ruins would be included aswell, the aftermath of the great disaster.

I recommend looking at Black Reach to get the idea.

i will look into this further whilst learning more about the creation kit.