A design that is almost impossible to make to stand upright. In addition, it is almost impossible to service the submarine in the port, as it will never be able to enter there.
In fact, I think if someone wanted to make a "hidden large submarine", then "below" they would just make another regular submarine of a much larger size, and not a similar "underwater skyscraper" ("bottomscraper"). While maintaining the same displacement, it will theoretically be able to at least approach the port, in addition, it will not have problems with vertical stability.