When I read lost-cities, Castle in the Sky (http://en.wikipedia.org/wiki/Castle_in_the_Sky) came to my mind...though I don't know how much that applies to you.

There is also Quest for the Lost Civilization...an old tv series. But I think your best bet will be Discovery Channel documentaries on Ancient civilization.

I also do have a question: how do they live on top of a lost city and not know? Is it all covered so no one noticed until now? Just curious, in terms of a plot holes.