The Underdark is a staple of the Dungeons and Dragons mythos. The Underdark was introduced in Gary Gygax‘s Descent into the Depths of the Earth adventure. It is an exotic environment populated with wicked and vile creatures, outcasts, and Cthulhu-mythos nightmares.
As a location, or even a concept, I have only rarely been inspired by the Underdark. It represented the penultimate dungeon crawl, and frankly I've always wanted more from my game.
Part of the reason my Night Below campaign collapsed is the shift to the Underdark; At the time, my previous long-term campaign was run completely off the cuff; Little to no prep. I would respond to the characters actions or introduce wild and crazy plot twists on a moments notice. I chafed at the the switch to what felt like tighter constraints (i.e. there were lots of tunnels that defined how to get from A to B). The first book of the campaign was of a wide open region, with points of problems.
I also found tremendous inspiration in 3.5E's Dungeonscape. I bought this book solely for the Factotum class, by far my most favorite 3.5E character class. However, it's hard to shell out money for just a character class. So I dutifully read through the book, and realized that there could be a lot more going on. And, I was free to turn the volume way up and dispatch with any pretense of believable; After all Beholders and Mind Flayers aren't all that believable. In fact this book started altering my mind.
I even went so far as to purchase on eBay Dungeon #70 for Wolfgang Baur‘s Kingdom of Ghouls, an adventure where all the usual denizens of the Underdark are fleeing from something truly hideous. From my reading, it sounded like the Kingdom of Ghouls adventure was regarded as one of the best written Dungeon adventures. Wizards of the Coast released Kingdom of Ghouls for 4th edition; It was stated as an homage to Wolfgang's original work.
But upon reflection, it is easy to see that the Underdark simply represents a different world that doesn't require a spaceship or teleportation to get to. The fact that you have to traverse tunnels is really immaterial; After all it's an imaginary world where rivers can change direction or disappear on a whim. The imaginary world need never see a drop of rain yet can easily be full of lush vegitation.
The other piece that dawned on me as I studied Math in college was that the Underdark, and it's smaller cousin the dungeon, are each a network graph. In reality this constraint is very helpful; Only worry about the nodes and edges of interest; The cities and roads if you will.