22 reasons why your fantasy RPG world has so many monsters infested dungeons

There are many different ways to involve exploring underground complexes filled with monsters in a roleplaying game. You and your group might prefer the straightforward challenge of dungeon crawls without much story to wrap the encounters together. Alternatively, you might enjoy games with complex layers of plots and a GM who tries to avoid anything too familiar.

Whatever your situation is you might like to have an answer for the question; “Just how come there’s another dungeon-like area to explore? Why is full of monsters again?”

This puzzle was put to a group of gamers and here are some of my favourite responses;

The dungeons are the remains of a large but fallen civilization, now long gone and forgotten to some apocalyptic disaster.
One or more gods of the world encourage their followers to build large, complex, underground temples and then move on.
All the dungeons are naturally occurring structures created by lava or rock-eating creatures.
The remains of a recent feudal kingdom, with many lords and ladies, each of whom had an obligation to build and maintain a dungeon.
Ten thousand years ago, an evil wizard traveled to the center of the world and built a storeroom full of enchanted abacus, which are linked together in such a way that they calculate out procedural expansions to the dungeon, manage the stocks of various monsters, and so on. It’s been at it for ten thousand years. A good chunk of the planet’s volume is now dungeon, but nobody on the surface understands the extent of the problem. The excavated mass, with no outlet to the surface, is mostly converted into heavy metals and used for bullets and decorations. In a few thousand more years, the planet’s whole mass will have been converted into dungeons, monsters, and magical weapons. This configuration takes up considerably more space than a regular planet, so the planet by then will have expanded greatly. All of the matter of the other bodies in the system will be drawn into the process until there is only one gigantic dungeon star. This is the natural life cycle of planets where conditions are favorable to the formation of evil wizards, and is the ‘great filter’ that prevents races from pursuing magic to the point of godhood.
The existence of The Underdark as a parallel biosphere means that there are layers upon layers of underground complex ecosystems.
They are a kennel for the monsters, keeping them locked away from humanity.
When I need a reason for a an underground dungeon, I lean toward burrowing civilizations. Typical fantasy Dwarves are a good example; they do all the digging themselves. You just have to figure out how to account for them being gone and monsters having taken over, which is also easy for more typical monsters.
The world is eternal, there is no beginning and there is no end. Or at least that is what the sages say.
Regardless, the world is very old and civilizations, as they do, rise and fall. One era, the orcs might rule dominant, the next the dwarves, then the goblins, maybe the elves, an era or two of human dominance, an invasion of demons the next. All these happen time and time again, and yet we only remember fragments.

As a civilization falls it leaves behind ruins and memories. A subterranean race tunnels to the surface, a mad wizard enslaves a race of burrowing creatures to enact his mana crazed dream of an underground utopia but dies before it is complete, a crazed demigod demands a hidden temple.

Time rolls on, civilizations, mad wizards and crazed demigods are eventually forgotten in all but myth and a few fragments. Yet, unlike structures built on the surface that decay with the weather and other forces of nature, these underground structures remain, barring the shifting of the earth itself and the more destructive mindless burrowing denizens.

With a history of tens of thousands of years, a world full of creatures who have mastered the art of moving stone, magics that can shift the earth itself, and many who even shun the light of day, how can the world not be riddled with ancient burrows, crumbling dungeons, lost crypts, caverns large enough to grow a small city, and all manner of subterranean habitat?
The monsters live underground their whole lives, they send heroes into the overground to take our stuff too. The dungeons are just the bits where the two worlds cross.
There were two ages of turmoil where demons tried to take advantage of the Gods being all messed up and started tunneling so they could attack major settlements by surprise. Like North Korean tunnels, there are a couple well known systems but quite a few more are suspected. The well known systems are obvious and some are safe enough to go unguarded with simply some keep out signs. Some have things that have made homes in them. There are a four dwarf families that run the majority of mining operations in my game world and they are keeping a few other networks under wraps X-files style. Then there are a few small areas that maybe the thieves guild uses to hide contraband and the like. The rest probably still have demons just waiting for the go order.
The majority of mine are underground temples to very old gods. Sometimes temples to newer gods have been built on top to mirror Christianisation of older religions.
I’m a fan of “local town overrun 10 years ago” because I enjoy using current, familiar settings, like a tavern, and turning it on it’s ear by making it bizarre and in disrepair with obvious signs of strange creatures being there.
An old and long-forgotten race used to summon and capture monsters from astral planes and even semi-deities, and they built huge mystical buildings to imprison them, e.g. to harness their power, or for any other reason.
The power of those entities is great enough to have imbued the building despite its magical protections, making it a beacon and a haven for lesser monsters of all kinds.
Purple-worm shaped iron golem, built for tunneling. There was at least one of these in my last world… until those joyriding Drow teenagers ran it out of the ground and crashed it.
Magical fallout from Spells of mass destruction forced all races into vaults underground for a hundred years. Society has returned to the surface but the vaults are still there.
War. You are an average citizen in an at-war country. You know that one day or the other some barbarian army is bound to come screaming for your blood and you can’t fight. Suddenly, digging a few hidden rooms in your cellar seems a good idea. Repeat for a century or two, let people forget and discover the refuges and adapt it to other things and you will get a plethora of small and nice urban dungeons.
Mine had teleporting genocidal magic-eating sorcerers who could only be warded off by large, expensive, difficult to cast rituals over cities. People would occasionally attempt to strike out and build a new city but 99% of the time the monsters would scry them before the ritual was finished and then bam! Ruins. Occasionally they’d try to get around this by building in caves or underground, in which case we’ve got a dungeon.
In our 13th Age campaign the dungeons are alive and dislike the fact that monsters want to live in them. The dungeons perpetually dig up so it can unload it’s monsters on the surface. This is of course bad news for the small village the adventurers happen to be in. To prevent more monsters of pouring out the adventures are asked to go in and kill the dungeon. After the dungeon has been killed it will start to sink back into the ground. After a decennium of resting on the bottom layers, it will resurrect and again begin it’s ascend. So the dungeon lives in dubio, it wants the annoying monsters out, but wants to keep some of the more dangerous once close to defend against invaders. Some dungeons might generate trapped rooms or shift it’s rooms around to bring the more dangerous monsters closer to the invaders.
Magic doesn’t seep into the world at a uniformed rate. Magic prefers places of memory, especially historical places long since forgotten. As magic seeps into these places, it’s infused into the objects that exist there. A sword used to slay the general of some invading horde 1000 years ago in a long abandoned armory attracts more magic than an unused one, for example. Mundane objects become magical over time. A well crafted sword may become a vorpral sword if used to cut off the head of someone important an it lives in a high magic area for a long time.
Some monsters like dungeons and other high mana flowing areas because they’re safe and forgotten about. Some monsters feed on mana. Goblins feed on mana in my setting, and as they are exposed to it they transition into hobgoblins. Dragons are immortal, and need to consume mana to grow. A horde of magical treasure with powerful memories attached to them means more mana which means more powerful Dragons. A Draconic horde is more than just greed or obsession, it’s necessary to grow stronger.

Adventurers go out and to recover high mana objects for mages. Maybe there’s a tree that is infused with mana, that some wizard can study. Maybe they find a magic shield. Maybe they find a sword that was passed down from generation to generation that protected a small village, and was the only weapon they had.
What about a physical/geographical change that occurs after/because monsters have moved into the area? Like a fungal growth that hardens into stone… or the softening of the earth which sucks down villages and towns?
Forces of corruption are constantly looking for ways into the world. These forces find weak spots and begin corrupting the environment creating monsters and warping the terrain in areas that are not occupied by the sentient races (and even that is not enough to always keep them out)