I think it would be cool if there were long tunnels inbetween, say, Balmora and seyda neen. (not magical doors)

Technically, the Underground2 has this, however, the mod has its bugs and isn't extremely lore friendly, but if you don't actually play the mod, and just use the extensive tunnel system strewn between many of the big cities you shouldn't have any issues with it. If you can handle a few ctd's though, the mod is quite entertaining in and of itself. Thats the only mod that I know of that has such an extensive tunnel system, though maybe someone more experienced with mods than I can answer with athority on the subject.

There is an underground/thieves lair mod for Balmora that has an underground apartment beneath the Stil Strider in Balmora with a tunnel to the Thieves Guild's club in Balmora. There is also a Dwemer mod that adds many 1:1 tunnels to the game. I think its called "Dwemer Tunnel System" or something.

I'm using a mod right now that includes various cave/ruin systems between towns, but I don't know for certain if they go under Balmora. I just found http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=5188 on PES, which not only includes the mod I aws talking about, but numerous other dungeons all underground.

http://users.bestweb.net/~pogo/stuff.html can be loaded together and you apparently can walk underground from Mournhold to Balmora to Vivec to Solstheim and more. These dungeons were procedurally generated, so they aren't necessarily the prettiest.

Edit: it's somewhat like the link Krovosos posted.

I have the Thieves Guild Tunnel System (follow the link to PES in my signature), which links together several locations that are/can be tied into the Thieves Guild. Apparently it has some issues that I haven't addressed (dirty refs, etc).. maybe I'll fix that someday.

I know, that is the mod that brought this to mind. however, that is not 1:1, "Super Vvardenfell Tunnels" is too high level, and too much loot, and "the underground 2" includes advlt content.

I've had several requests to make it a 1:1, actual-distance kind of deal, as you can see on the download page. It was meant to be a quick travel, not an attraction for purists. I guess that I could have put a script on each of the doors that advances the time according to how long it would have taken to travel that distance on foot.

That's a good idea! If you ever get the urge to work on your Thieves Guild Tunnels again, I'd certainly be interested in downloading it.

Hmm... using ES search, there's also http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=5188. I've not used it, so I don't know exactly what it does. http://search.yacoby.net/?name=&author=&description=tunnel&game=MW&page is the results page for "tunnel."

I use these, and tbh, despite it being automatically generated, there are a few interesting areas in there, and I had a lot fun dungeon crawling in there. The main problem is lack of path grids really.
Use NoM and TLM, then gather up your supplies, and dungeon crawl from city to city, it's fun as hell. :hehe:

I spent like, what, 5 levels down in Knaarus? Seriously, those dungeons were an amazingly fun romp. However, I couldn't play through Lob Shelepi; Not only were the Tribunal enemies unfairly hard, but it made no sense for them to be down there.

Krovosos, gtfo, logical thought has no place in dungeon crawling!