here is only a sewer system under the Imperial City in plain Oblivion. This is a real pity, because sewers can give a real boost to the reality feeling of the virtual world. Sewers are known to harbour criminal types (just as every cave in Oblivion seems to be infested with bandits), and with this idea in mind, the mod 'Underlife' was created. 

By adding sewer systems to Bravil and Cheydinhal, complete with NPC's that live there (beggers and the sorts) and monsters that roam the tunnels (rats and other little nasties), there seems to be a whole world living under these cities now. Normally we don't add mods to the site that are still in development (alpha or beta versions), but this mod is so cool and in fact completely playable, we decided to add it nonetheless. Better yet, we voted to make it 'mod of the day' ! The only thing that could be added is obviously the sewers for the other cities, and I guess that's why it's v0.4.2.