I'm sure this question was raised before, but still: Why in the manga are all the citizens so terrified from bursting the bubble around the city? It's not like they will drown, actually they will be able to move faster and better in water.

And related to that, why was this city built that way in the first place?
If I'm not mistaken it was built before any thoughts of making contact with humans, so why bother creating a protective shell for creatures who are not there?