Over at Howling Tower, Steve Winter posted his Kobold Quarterly column regarding that venerable D&D locale, the underdark.
When I read it, I realized that I'd fallen into the same trap that many a DM has been victim to, cutting and pasting the Forgotten Realms underdark into my home campaign without a thought.
It's an easy thing to do, and in my experience, it's what players expect. This is precisely why no DM should ever do it.
With all the time I've investing in crafting a the world above ground, why shouldn't I do the same with the subterranean elements? I'm lucky in that my players have yet to venture into the earth. I've established that drow exist, and worship a spider demon, but other than that, my canvas is still blank.
I could easily make drow a warlike race of raiders that have been driven to the fringes of the subterranean world, which is why they sometimes raid the surface. I'll probably spend some time digging through the monstrous manual to get ideas for what sort of creatures inhabit das unterworld.
And by "inhabit", I mean what nations struggle against each other below our feet.