I've been working on an rpg setting, tens of thousands of years in the post-technological future, after several major waves of civilization have arisen and disappeared, through disaster, singularity, decay, ascension, etc.. As an anthropologist with a strong interest in archeology, I'm fascinated by the idea of dead civilizations, and the idea of multiple layers of civilizations known only by fragmented legends I find a very romantic image. (In practical terms, it will probably mean a lot of dungeons and lost cities to explore. *sigh*)
So I've been doing a bit of research on what artifacts may remain...what things from our time period might survive 30,000+ years?
As it turns out, not much. the first authoritative source on how cities that aren't maintained go, is here in New Scientist:
Return to Paradise
After that, I've checked my old usenet newsgroups, and found some interesting threads on the topic.
Survival of Structures
How Do the Remains of Civilizations Decay?
What will Last 20,000 Years?
So I've got something to work with for the basic artifacts of civilization that might be still around. Now all I have to do is pepper the landscape with the occasional weird high-tech structure that's more-or-less indestructible, and who's purpose is forgotten. Lots of romantic, morbid fun.