Thieves Guild hideout.
(from old forum thread)Use the shell for location of entrances. The uppermost entrance should be the most used one.
I imagine a few members living down here, mostly wanted people. The area is also used to store loot, make plans etc.
Claimed by RyanS, but abandoned since then. Use his latest update to work from, or make something completely new if you'd like so (as long as you keep the two entrances in their place!)