Thanks for the feedback!
One of the things I really like about the whole ship concept is that it could go in so many different directions.
Some of those other modules could be (and probably will be) other cities. Some of them could be warehouse or factory levels, dangerous training levels, or special scientific levels where rare materials are grown/cultured for harvesting.
Pyrion, you are correct that there is a lot of room below the main level to the bottom of the module. I just measured it, and at the current design, it's 890 feet from the street level to the bottom of the module, so that's 271 meters.
If we assumed that something like a maintenance level had a hallway ceiling height of say, a minimum of 10 feet to a max of 20 feet, we could theoretically fit as many as 44 underground levels going straight down before we reached the bottom of the module.
Of course, special levels could have much higher ceilings depending on their purpose, but that's a lot of space to play with!
Most likely it would be a mix of more traditional maintenance, waste disposal levels etc., along with more unique, larger areas mixed in.