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. The museum was apparently having a feature on the mythology of Hell and the afterlife during the time of the Great War, as most of the remaining exhibits (including the name of that section of the Museum as well) are all themed as such.

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.

Charon, the grouchy thug at the bar, is also from fabled Greek mythology, being the the owner of the boat which takes dead souls down to Hades.

Charon is an NPC who can join your quest under specific circumstances.