There's been a lot of fantastic discussion lately in regards to Ye Olde Megadungeon, and it definitely got me thinking about one of the projects I had on and off been working for my homebrew.
By way of explanation, let me state that as an old Rifts fan, I firmly believe two things:
1) There's no weapon, item, or tool that can't be made better by adding "plasma" in front of it, and, more central to this post,
2) Mega is good. A couple of Megas is better.
I basically wanted a huge, huge underground series of works--sort of a highway connecting a large number of dungeons (perhaps "Macrodungeon"?). I didn't want something as open-ended as the Underdark, but of a more refined original purpose. I tried to work out something fantasy-plausible for this design, and this is what I came up with:
The Dwarven Darkways were the crowning glory of the Dwarven Civilization. While the races of Elf, Man, and Orc warred and braved the monstrosities of the unconquered wild on the surface world, the dwarves carved an incredible series of underground network of roads to connect their Citadels and to bypass problematic overland travel. Century by century, millenium by millenium, they refined and expanded the natural cave systems under Irrin into well-crafted series of subterranean roads and highways.
At the the height of dwarven power, this network was a sight to behold. Beautiful stone bridges crossed underground rivers; clever stonework and tunneling avoided and closed off those areas where monsters were known to lurk. Dwarven patrols in shining mithril mail protected the flowing trade between the Ccitadels. Glowing, cunningly-set Moonstones gave a cool, welcoming light overhead at regularly-set intervals. Gnomish trade routes linked into the larger highways. Picks rang out, hard at work on a hundred different mines and projects along the way. Tolls were collected, and the hundreds of miles of tunnels stretched between the midlands and all the way to the strange lands bordering on the western seas. The Dwarven Darkways were the marvel of the world; it was said they touched as many lands as did the overland sun itself.
The Citadels themselves were things of wonder. Extensively huge underground fortifications, they bustled with the trade of the dwarven peoples and a half-dozen underground allied races. The beauty of their twisting tunnels belied a purposeful design of sturdiness and defense. Their chambers sprawled in a myriad of directions, and all of it connected by the Darkways.
But, as with all works made by mortal hands, the Darkways fell from grace. The dwarves, always fewer in number than the tallfolk races, could not hold their empire against the twin perils of outside aggression and inside avarice. Fallen to infighting amongst themselves, many of their Citadels were abandoned or destroyed. Some lie empty, the years doing nothing to stale their infinite majesty. Others are haunted by a host of fallen dwarven warriors, unable to sleep in the ruined might of what was once theirs. Still many others were siezed by brutish races, such as orcs and kobolds. And some came to be inhabited by creatures far worse.
Today, some passages of the Darkways (especially those near the remaining Citadels) are still used and patrolled by the various remaining dwarven clans. More commonly, miles upon miles are populated by nothing more than wandering monsters and bloodthirsty subterranean humanoid tribes. It is said one could still traverse nearly the entirety of Irrin, marking the progress by each ruined Citadel explored. The remaining Moonstones still given off a now-erie glow for the daring explorer or traveler. There are paths down there untrod by mortal feet for many centuries now. But there also almost certainly remains an untold amount of treasure--the treasure of a world all its own.