|Both Treasures and Death await inside them.|
Dungeons are living structures that dive deep underground. They accumulate Mana, mix it with the surrounding's Life Force and use it to create Monsters to protect itself, those monsters are completely mindless and their purpose is to protect the dungeon from intruders... However, as the dungeon creates more and more monsters, they start going out and cause destruction in their surroundings.
Sometimes a dungeon accumulates way too much Mana and then suddenly creates a huge monster outburst that starts devastating everything in their path. It was one of those outbursts that made The Hero fight through many years to unify the nations that became Ternus and push the monsters back to Dagon's Dungeon.
Also, dungeons are usually named after whoever discovered them.
Dungeons are complex labyrinths that go deeper and deeper underground, any Adventurers that wishes to dive into them is recommended to bring a map and supplies to endure the long voyage.
While the labyrinths don't change, monsters are constantly being spawned to fend off intruders, so one needs to be always be careful of their surroundings as they explore.
In the deepest layer of the dungeon, a core that is filled with mana exists, it's generally protected by the strongest monsters the dungeon can summon. Breaking it would kill the dungeon and stop it from ever spawning monsters again, taking it away on the other hand is generally not a good solution as the monsters will keep being spawned on the way back, and the dungeon can always try reforming by creating a new labyrinth from scratch.
The monsters created by the dungeons are purely made of mana and aren't exactly living beings, they certainly do look alive, but it's pretty clear that it's not the case by the fact they dissipate into mana when they die. Dungeon monsters are somewhat similar to Golems in a way, since both are mindless beings with a singular purpose, but can move mostly on their own.
Whatever you find in the dungeon tends to mimic the beings of the surroundings, probably because the dungeon uses them as reference to create its own monsters.
Also, the monsters that leave the dungeon tend to acquire a more solid body structure that gets mixed with the energies of the outside, so they don't disappear on death like the ones inside it, making it pretty hard to differ monsters created by a dungeon and that were born outside.
One of the biggest reasons adventurers dive into dungeons is because it can be a very rewarding endeavor. To begin with, sometimes a monster doesn't dissipate properly and drops a Mana Crystal from its body.
That is only the tip of the iceberg though, the real enticing part of dungeon diving are the magical items. In order to increase the power of its own monsters, dungeons have the habit of creating magical items to be wielded by some of its monsters, and those don't disappear when the monster die. Killing those enemies and stealing their items can be a great source of profit or a way of increasing your fighting potential by having a new strong item to help you.
Not to mention that the body of any adventurer that died in their expedition can also have some things worth looting.
Lastly, the dungeon's core is one of the best and most rare materials for Alchemy, even if it's generally delivered as broken shards, it's still an incredible and very expensive material with what is probably the biggest mana concentration in the world.
Those are the dungeons the Adventurer's Guild found so far and that they have available for exploring: