STAY VIGILANT, LOOK FOR THE EYES OF DWARVES
Abandoned buildings left unused for too long grow grow weedy, dusty, strange. The angles twist and the geometry buckles under the barometric pressure of anti-life. Among the dust and cobwebs, traps blossom. A brood of goblins rise out of the earth and shake clods of birth matter from their heads. Exotic, threatening beasts settle down and nest; below these lairs, trap doors lead to newly-formed but entirely ancient and archetypal stairs, dank tunnels with torch brackets that never held a torch.
Sewers have to be regularly patrolled, newly-budded secret doors smashed and burned. Behind these doors may be shimmering teleporter mist, writhing, glistening gristle or simply mundane wall. The door is destroyed, what lays behind sealed away under rocks and incantations.
Dwarves have a sense for these things, when ancient edifice or natural cave is about to "turn," dwarves appear, clear the dungeon (or die trying), pull up the root and leave Delver's Eyes to mark where they've been. The lashes and pupil of a Delver's Eye indicate certain facts like, "No dungeon here in two years," or, "Goblin nest burned last winter solstice," or "this dungeon cleared by the Bronze Fork." If you know dwarven runes or if you can cast read languages, this is all apparent to you. Dwarves will teach the Eye to others that commit themselves to the cause of dungeon eradication.
Dwarf clans left too long alone in dark become fanatical Duergar, monomaniacal dungeon-clearers. Dwarves live a long time, but Duergar are seemingly immortal, though they grow increasingly old, decrepit and insane. Duergar may watch a dungeon "grow" or even "clear out" settlements to make room for a suitably large dungeon, just so they have something to do or because they believe only the "big ones" are suitable for their own capacities. Know them by their cruelty and their madness.
Some few dwarves may instead fall prey to the whispering of a dungeon far below, slowly becoming Derro. They may befriend the children of the dungeon, or they may keep to their own. Derro colonies often devote themselves to deep dungeon biological studies, creating machines of strange and enormous power, machines that run on the nightmares of an entire city and must be placed deep in the city's sewers or which must be fed all the works of an entire civilization to create an army of firmir-golems, all of whom are haunted by memories of the art they once were. Most of the Derro's creations are incredibly powerful and often quite beautiful. While the Duergar eschews the devotion to craftsmanship common to many dwarves, the Derro are possessed of finely refined aesthetic sensibilities. Know them by their black, paper-thin skin and enormous eye.
When travelling through an abandoned place and you do not see the eyes of the dwarves or explorers that went before, beware or turn back.
Maps sold to travelers are concerned less with relative distance than keeping accurate records of which Delver's Eyes the plot safest passage. Explorers will find their own maps valuable, especially if the maps provide relative safe passage to something useful or valuable.
THE DAYLIGHT PEOPLE, THE PEOPLE OF THE SUN, THEM THAT SLEEP ABOVE US
This could be just humans, or whatever. Humans, like, work really well, because:
The impulse of the human being en masse is basically: multiply, civilize, colonize, dominate, cultivate, as if the impulse to expand, build and civilize is a design feature. As if human beings are something's solution to dungeons and monsters because: dungeons and monsters are virulent and metastasizing but they can't really grow in the presence of a human.
THE PEOPLE OF THE SUN ARE TOXINS IN OUR BLOODSTREAM
The presence of a single Daylight Person causes dungeon growth to slow to a crawl if not stop completely. The dungeon is sluggish, confused when humans wander through it and it begins to wake monsters, set traps, and otherwise expel its guts to purge and frighten away the intruder (like a sea cucumber).
Dungeon entrances serve as cloaca, both mouth and anus. Dungeons swallow adventurers and belch out orcs. They fart great clouds of goblins and excrete gnolls, ogres, giants and titans. Plenty of dungeons hide their true cloaca by budding off lures, like angler fish casting in the deep (Acererack's tomb has two lures, even).
What's the relationship between these people and the dungeon? It's not clear. They're a bit like an antigen, and they're a bit like a spore. Other People long-since separated from a dungeon will talk about other worlds and other places. Maybe they hail from subterranean civilizations lit by strange suns, maybe other planes, other planets. These New People that can remember their places of origin miss them keenly and are often morose. Few are capable of reproducing, even fewer are interested in reproduction.
All are highly susceptible to the ambient environment. Orcs left too long in swamps grow moss, ooze poisons, goblins left in plague zones strain against a load of ominous buboes.Giants left in caverns deep are half dirt and half rock. Driven by a mostly stupid, half-god will, crawling, digging slowly through the deep like enormous worms. Their bodies serves as sieves, collecting ants, worms, centipedes and every other thing of the earth and passing it through its dirt until it emerges different, larger, vicious, mandibles dripping venom.
Giants sleeping atop mountains grow trees, put down mineral roots to lure miners, explode in volcanic fury or else elongate and dissipate into the ozone to flood crops and carrying swarms of strange, dancing insects whose touch causes outrageous violence.
Most giants with physical form are partly hollow, gestating dungeons, growing to accommodate the inner structure within.
A titan is a being of cosmological existence, is essentially a megadungon stretched between multiple planes of existence. Most sleep and grow and never wake; they are congenitally unable to survive waking existence very long as they are simply too large and impossible, their constituent, generative magics unable to sustain them upright for very long.
As a waking titan rises from the ground it shakes towns off its forehead and sweeps a city from its belly button. Most collapse at birth, moaning, crumbling, shedding wizard's towers and evil cleric's keeps, flinging dungeon seed hundreds of miles away in their death throes. A few make it to the standing position, their head and shoulders fully piercing the membrane separating space and planet, their head spewing dungeon seed far into the beyond and perhaps becoming a doorway for other, stranger people.
Other planes burble and pop in a titan's guts; their brains are a massive clutch of ab-reality (beware the places where wizard's tower gather in unnaturally close proximity for they likely plumb and quarry a titan's mind below) and they shed monsters constantly, like dead skin cells.
The Other People hear dungeons; they close their eyes and feel the thrum of the dungeon's blood. Its heartbeat is tremendous, throbbing under their skulls like a final migraine. An Other Person kicked into the depths of a dungeon will tumble, jamming hands to ears to stem the growing throb, becoming orc, gnoll, troll, ogre, giant even as brain and blood vacate their skulls and their bodies return to the dungeon's original matter: hatred, madness, cave mud and worms.
At nights, around fires, the Other People sing the songs of the dungeon, it's first incarnation (was not-stream bed), it's growth (oh cube, jelly belly), it's first kills (farmer-digs-too-deep), its mazes, its traps, its first lich. They sing of their new selves: atavism, cannibalism, an occult tumult into chaos, madness, the antidote to creation.
Sometimes a new-born dungeon simply comes into being complete with traps and monsters. In these first moments of their new life, before the dungeon's song has truly taken hold, Orcs draw blasters and peer in confusion into the dark, all are confused, in pain, frustrated, frightened or else enrage. And then the dungeon's life becomes their own and they blink, and set about digging latrines.
The archives of a lich-explorer contain reports of watching a dungeon-becoming, the sandy floor fusing into stone, the beholders unfurling from a few rotting pillbug corpses to swivel eyestalks in panic, the pit fiend bellowing far below.
The larger the creature, the harder it is to shed the dungeon's hold. Goblins independent of a dungeon are civilized in months. In generations, they no longer hear a dungeon. Orcs take longer. Ogres are never properly, fully civilized; their minds unable to totally adjust. Giants and titans are carry dungeons within themselves or are dungeons, never "away" from a dungeon.
THE CLEARED DUNGEON
Has to be civilized (turned into a hideout for characters, converted to a mine, used as a storage cellar for a keep) or systematically pulled down. Some municipalities, those far from the frontiers, retain sappers for the purpose, but most frontier settlements need help keeping the dungeon clear. Every room must be explored, cleared of treasure, traps and monsters, every secret door unlocked or else the dungeon will slowly reconstitute itself, perhaps where it was, or perhaps somewhere else. It will likely return meaner, with foreknowledge of the characters and their methods.
THE LOST AND THE DREAMING
Dungeons dream in forgotten places and long to be born. In the liminality of the taking form, the dungeon's dreams and fantasies blow like a hot breath from its hiding place and cause confusion and nightmares. Where abandoned sewer lines and city intermingle, usually in the poorest places, dungeon birth is foretold when the poor suffer from madness & mutation, plot riots and insurrection.
Dungeons also appear in uncivilized minds, in items of power too-long unused. Wizards have dungeon-bent minds, cultists, punk priests, chaos Catholics all find themselves compelled to live by nascent dungeons, total capitulation to the dungeon and it's reigning deities. Wizards summoning monsters are essentially just wizards peeling back the difference between this place and the cloaca that first ripped open their minds.
THE MEGADUNGEON, OTHER PLACES, OTHER HELLS
Effectively, every dungeon is somehow connected to every other dungeon by a network of ideas, deities and tunnels. Some dungeons are too small to grow fully navigable tunnels to other dungeons and transport between them requires acts of veneration and sacrifice to the governing malignant deities. Dungeons are also connected by certain conceits, the dream-contents of the dungeon made coherent: the sulfurous dungeon of precarious rock dancing in churning magma is connected by dream-song to the volcano dungeon which is likely connected to the hotter hells.
The more potent and dungeon-minded traveler simply need to close their eyes and fill their minds with the entire song/spell/dream of a dungeon. The more broken and dungeon-bent the mind is, the easier such travel becomes.
Older, deeper dungeons bud misdirection- sublevels or full on symbiotic relations with stranger planes, alien planets and other hells.
pictures: Dali from Spellbound, Blake, Zumart and Blake Again