[... Hiccup it is not a bubble it has a forest and a cathedral and lots of little castles on top of it. It is an UBERBUBBLE.]

Much of the main body of it is filled with a chemical that is lighter than air. Thus, the chemical rises within the body of the airship and provides lift.

[Was that simple enough or will he have to think of another way to explain it OH THE JOYS OF HAVING A VIKING SON]