Underworld

The Underworld is an area where the ostracized sapient ghouls (as opposed to their feral brethren) 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, however, as they tend to shoot Ghouls on sight.

It is located inside the Museum of American History. The museum was featuring an exhibit 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 Underworld.

One of the robots in Underworld is named "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. (Cerberus is a Mr. Gutsy robot, which has three independently mobile sensor modules.)

Charon, the grouchy thug at the bar, also owes his name to Greek mythology, in which Charon is the pilot of the boat that takes dead souls across the underworld river Styx into Hades. This is one of many references as well to Dante's "Inferno" which Underworld, its circles, and many of its characters are based off of. The game incorrectly cites "Paradise Lost" as its source for the exhibit, a book that focuses very little on describing the underworld.

Charon is an NPC who can join you.

There is a ghoul in the back of the top floor, main room, who has clean hair, unlike the other ghouls. He will give you a free haircut. This is because he says that other ghouls pay no attention to their appearance, and he is dying to work on someone who actually has hair.

NPCs

 * Cerberus - A Mr. Gutsy robot that protects Underworld, but hates the "zombies". With Robotics Expert you can remove the Combat Inhibitor and allow him to attempt to kill everyone in underworld. No one seems to notice or care about this once they've killed Cerberus.
 * Winthrop - The mechanic of Underworld, he keeps everything in working order as much as possible. He will barter 5 Scrap Metal for 1 Rad-X, Rad Away, or Stimpak.
 * Tulip - a general trader with an interest in the history of Underworld. She gives you Paradise Lost when you talk to her.
 * Patchwork - A drunk that wanders around town.
 * Ahzrukhal - Owner of The Ninth Circle and Charon's contract. He also sells drugs.
 * Charon - The Ninth Circle's bouncer who will join you if you purchase his contract.
 * Carol - A pre-war ghoul, she is the owner of Carol's Place and the (adoptive) "mother" of Gob.
 * Greta - Runs the diner in Carol's Place
 * Snowflake - A barber who will change your hair for free just to have a chance to work on an actual full head of hair.
 * Doctor Barrows - The doctor in The Chop Shop, he will heal you and sell medical supplies. Can also revive Reilly if you need him to.
 * Nurse Graves - Assistant to Doctor Barrows.
 * Reilly - Found lying on the bed in The Chop Shop, you can revive her to start the Reilly's Rangers quest. She will be found in the Ranger Compound once the mission is complete.
 * Mister Crowley - Found in Carol's Place or The Ninth Circle, he will hire you to kill 4 bigots to start the You Gotta Shoot 'Em in the Head quest. Once you give him the keys, he will head for Fort Constantine, and may hunt you down if you already took the armor.

Related Quests
Recruit Charon You Gotta Shoot 'Em in the Head Reilly's Rangers (quest) The Wasteland Survival Guide (If you chose to blow up Megaton)