If 3000 meters in a cylindrical height alone,

 

the diameter of the cylinder would be immense....

 

and so the ship itself must be.

 

 

If however, cube, eventually joining together

 

from within, whatever constructed...

 

 

Sigh....

 

 

Then the previous idea of cubical valves,

 

for creating the "sea city"

 

is the only way to go,

 

to create a large hollow space from within.

 

 

Attaching the sides to it as well,

 

for stability....

 

 

Sorry, thinking about it, it's far more monumental than

it would seem......