[That's right. Dungeons are a good place. Because the girls don't wear make up, you can tell whether they are pretty. Regardless of a swordsman or warrior or a magician, they have really good bodies.]
Putting the scumbag comments aside.
According to the books, dungeons are a form of magical creature.
It is merely a simple cave that has warped due to the accumulation of Mana, and finally changed to a dungeon.
At the deepest part of the dungeon, there is said to be a source of power, a Mana Crystal, and there is a protector that protects it (Boss).
The Mana Crystal is a bait that has a strong temptation.
The magical creatures that are drawn into the dungeon, will die from triggering a trap, or starve to death, or killed by the protector.
The dungeon will absorb the Mana from the Magical creature.
But, there is also the possibility where the Magical creature eats the Mana Crystal, or occasionally gets buried when the dungeon collapses.
Dungeon sounds like a Magical Creature because of this fuzzy description.
Also the Magical creatures that are not the only ones drawn to the Mana Crystal.
Humans are also foolishly drawn to it.
It seems that the Mana Crystal can be used as a catalyst for Magic, so the price for it is extremely high. Even though the price is set according to the size, even a small Mana Crystal will sell for enough to play around for a year. Furthermore, while the Magical creatures are only eying the Mana Crystal, humans aren't merely going for that.
There are Magical creatures that has years of Mana in them, or there are adventurers' equipments that have been left behind.