Underworld

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 and Roman mythology, in which Cerberus is a multi-headed dog which guards the gates of Hades, to prevent those who have crossed the river Styx from ever escaping. (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 was the ferryman of Hades who carried souls of the newly deceased across the river that divided the world of the living from the world of the dead. Charon is an NPC who can join you.

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. If you have a high enough sneak skill, you can also hide and shoot off the inhibitor, and no one seems to notice you as they kill Cerberus. 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, telling you that the afterlife exhibition in the museum is based on Paradise Lost, a book about 'a guy who goes to hell'. This is a mistake as Paradise Lost is an epic poem that concerns the Judeo-Christian story of the Fall of Man; the temptation of Adam and Eve by the fallen angel Satan and their expulsion from the Garden of Eden. Underworld is actually based on Dante Alighieri's The Divine Comedy. The first canticle is Inferno (Hell), in which all of Underworld's reference's come from.
 * Patchwork - A drunk that wanders around town and mainly provides humorous dialogue.
 * Ahzrukhal - Owner of The Ninth Circle and the holder of 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 with a speech check.
 * Nurse Graves - Assistant to Doctor Barrows.
 * Reilly (human) - 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.
 * Sydney (human) - An NPC from the Stealing Independence quest, who originally appears in the The National Archives. If she survives that quest and it is completed with rewards, she will journey from Rivet City to the Underworld via the Anacostia Metro Station. If she survives the journey, she will be found in The Ninth Circle selling armor and weapons.
 * Moira Brown - The woman who runs Craterside Supply she will appear in the underworld if you destroy Megaton. (You have the option to tell her you made her a ghoul if you find her around Megaton after you detonate the bomb, she also will sell things and continue the Wasteland survival guide quest there.)

Related Quests

 * Hired Help
 * You Gotta Shoot 'Em in the Head
 * Reilly's Rangers (quest)
 * The Wasteland Survival Guide (If you chose to blow up Megaton)

Trivia
The large ruined painting on the second floor of the Concourse is by William-Adolphe Bouguereau.