Lost and forgotten world

[fyifoff.livejournal.com]

I posted way back, and have not been able to get back here; So here I go:

In my Sci-Fi there is a lifeless planet, much like a desert. I boy and a girl live in a dome which is made man. They leave because of a comet, which causes a storm, this time however the storm removes the sand. Their dome, is atop a 10 story building, but the 'real' city is underground, of which, one solo survivor is still alive.

My problem: The city isn't really dangerous, yes there are two different groups after them and whatever they can find in the city. But I can't find a lot of movies that cover finding lost cities. In most movies, these unseen underground cities crumble and is lost forever. That is not happening here.

Other then character arguments and having to figure out a 'puzzle' to get by a location what else can I add??