Environmental challenge anyone?

[iCi]

An environment on a foreign planet that contains organic life form and a structure built by the life that lives there (just a generic example).

Its a genuine idea, maybe we can narrow it a bit more, like, for example - they use gas that is lighter then their atmosphere and a light building material to make their structures airborne, and the gravity is maybe weak, and maybe they can harvest that gas from floating plants that feed off the strong sun's energy that is stronger where the atmosphere is not so thick (far from the ground) and so on...

Everyone can add a little bit and the creator of the challenge can take those information to make a logical challenge :)

Also, we can specify what is it for- a book, game, movie, comic....