content: The Little Mermaid's bedroom.  Seriously though, the cutaway doesn't make much sense. It seems that it would more likely be a tube-within-a-tub design. Which would allow for a gap between the tubes where pedestrians could travel in an emergency and which would allow emergency personnel to get to a problem. It would also help protect the tube from damage caused by accidents - you could repair a section of the inner tube without worrying about flooding the entire thing.