Lost World Above
There's a book I read when I was much younger that told the story of a modern man trapped in an underground world, hunted by the evil fey who live there and unable to cross back to the surface world. My google-fu is not strong enough tonight to track down the title, but the book had a memorable beginning. The story's hero first discovers the underworld after pursuing a strange figure into the basement of a building, and then on through a gap in the wall of the adjoining basement, which is connected to yet another basement and another, following a secret highway connecting the dark corners behind furnaces and broken furniture. Eventually he finds a lost world deep beneath ours.
What if the lost world, though, was the one above? Two of the iconic races of the underdark, the drow and the deep gnomes, are first collected in the Fiend Folio, and there are many other strange deep-dwelling creatures in the book--meazels and meenlocks, dire corbys and doombats, gibberlings and grimlocks. What if their caves and subterranean cities were all of the known world?
Travel in this world is hard--even well established trading routes are treacherous. Bullywug bandits gambol and hop from their sumps to waylay caravans. Yellow musk creeper clings to stalactites and stalagmites, choking entire caverns like kudzu, a vile weed spreading its seedlings on the backs of its zombie victims. Mites and snyads pilfer and loot the unwatchful, and the loathed jermlaines use their nets and clubs and pitfalls to capture travelers for ransom, or worse. Beyond a few miles teleportation magic is risky for most spellcasters, but the enigmatic crypt things form a wealthy cabal of travel brokers, collecting hefty tolls to move folk over long distances. Their depots are encysted in solid rock, sealed off from all other tunnels and warded from scrying.
There are no large nations in the underdark, only small unfederated kingdoms, city-states, and trading centers. The drow and the other elvish races are a fading people. Their great halls of polished stone and marble are still, their galleries empty save for echoing footsteps. Dwarves mine alongside the svirfneblin for gems and precious metals, and the duergar study magic in squat, daemon-guarded towers. Deep halflings, the derro, are xenophobic nomads by and large, tending herds of cave oxen. Humans are more cosmopolitan, mingling even with xvarts, norkers and other humanoids in their cities and bazaars.
Forking off from the known routes are uncountable side caves. The further one follows these corkscrewed tunnels the stranger and more dreamlike the underworld grows: Two revenant princes marshal death knights and fouler undead in a perpetual war, each seeking justice for a betrayal long forgotten. Neither can ever truly triumph or fall, for the victor moulders away even as the vanquished rises again....Forlarren, the King Fallen and Redeemed, broods on his throne, black jealousy heavy about his brow. A network of skulks spies on the King's beloved and his subjects, and it is feared that the King may slay his new bride as he did the old....Elsewhere, myconids and algoids tend sprawling mushroom gardens, ruled by the Protean, an ancient fungal mind driven mad with the magic of the dozen wizards and priests it has enveloped....