A vast, interconnected chain of underground tunnels, reserves, and rooms. They were made so long ago that their purpose is now unknown, and large sections of them are used as livingspaces by Monsters that have been forced down there by Humans and don't want to be detected.
Thanks to people living in them, there are some areas within the bunkers that contain gardens, murals, and other signs of life. One may come across stretches of tunnels or hubs with many homes or facilities carved into the walls or built out of wood and scrapped metal. Not all of these places of activity are lit.
Many nocturnal and cave-dwelling animals also live here as pets and wildlife. Certain stretches of the bunkers are considered dangerous because of particularly territorial or otherwise dangerous animals.