Underworld

The Underworld is an area where the ostracized sentient ghouls (as opposed to their feral bretheren) of Washington, DC live. It exists as a small city, complete with a bar, inn, clinic, and "stylist".

Most humans are tolerated as long as they do not cause trouble. The Brotherhood of Steel are not welcome as they tend to shoot Ghouls on sight.

It is located inside the Museum of American History.

If you decide to blow up Megaton then Moira Brown will survive and come to live in the Underworld!

One of the robots in the Underworld is called "Cerberus" who jokingly is referred to as the "Guard Dog". This is a reference to Greek mythology, where Cerberus was the 3-headed dog who guarded the gates to the Underworld.