Dungeons are pockets of the chaos world that are connected to the human world through gates. They generally contain a horde of monsters, which are led by a boss, and remain open until the boss is killed. Once the boss is killed, the dungeon will stay open for about an hour before closing on its own.
It should be noted that in order to prevent the loss of human life as much as possible, then a dungeon must be cleared within seven days. If it is not, then it will undergo what's called a dungeon break, meaning that the monsters inside the dungeon will be able to cross over to the human world and attack the civilian population.