I'm continuing with my amazing skill at writing vague summaries that make the story sound way cooler that it probably will be. I was just doing a little scribbling, trying to get the backstory and setting worked out, and I came up with this:
The Lower World was uncovered some years ago at the bottom of a mine shaft. It's an endless labyrinth of tunnels, marked by ruins of some forgotten civilization. The inhabitants have long since vanished.
Or have they?
Have they just moved further down into the darkness? What still hides in the shadows?
Several expeditions were sent, led by Professor Harcourt "Harry" DeWitt, in an effort to map the Lower World and learn of its former inhabitants. On one such journey to the highest city, not far beyond the modern, human realm, DeWitt even brought his preteen daughter, the adventurous Catherine. She grew up among the ruins and the artifacts, learning the lost language of Subterranea.
And now, on a recently funded expedition to the lower reaches, the professor and his team vanished. A rescue team found nothing, and risked becoming lost themselves.
So Catherine, now grown and an explorer in her own right, must seek out a man with unusual skills, if she ever hopes to find her father.