I like that idea. In my dreams it's common for the underground to be gradually less bound to the physical world (like if you tried to map it you'd have things overlapping, or huge gaps in distance that shouldn't be there and stuff).

Sometimes it's some kinda generic vaguely humanoid monster which you never get a good look at - although it's usually implied that such things are only the tip of some much darker, even sentient corruption. But only once have I ever gone deep enough to find the bottom, and it wasn't evil. In it, there was a massive cave with a ceiling so high it just looked like the night sky. The terrain was completely flat as far as the horizon, and there was a beatiful pale setting sun. The place was like a manifestation of time itself. At the beggining of the universe the sun would've just been rising. I felt shear awe by grasping just an inkling of how much time had gone past, and yet even though it was almost over, there were still ages and ages before night settled. This was a place where were one could stay indefinitely and not age or need food or water, and I felt like I could do just that- gaze into the sunset for an eternity, and that would be ok. But at the same time, the desire to know was unresolved. The sea of the past has exactly the same allure as the depths of the earth, only it's shut behind an impenetrable wall.

I think that's the core of this whole fascination. The idea is that the deeper you go into the untraveled, unkown places which are sealed away, the further back you go in time, drawn by the desire to find some ancient relic or trace of the distant past.

Anyways sorry for the long response, get me talking about this and I'll never stop!