Fallout 3

It's a series of undeground metro tunnels, sewers etc. that run under most of DC. You often have to use them to get around as there is no way to reach all areas from above ground.

Ghoul city in Museum of History. Ghoul cities in all the Fallout games are affectionatly called "Underworld".

As the others said, it's a Ghoul city, in Museum of History in Washington D.C.'s ruins, it is just to the left of the washington monument (left from exiting the monument) it's very nice, plus there is a good vendor, a good bar, barber, inn and the owner of the inn is Carol, the "Mother" of Gob, who is the barkeep in Megaton, she has some cool options to talk to her with. o, yes, it also has a pretty cheap clinic that has an NPC who starts a quest called "Reilly's Rangers" in it.

its a magical land of faries and dust mites no just kidding its the only all ghoul city in the game and its kinda easy to get to just go in the museum of natural history and keep going until u get the a door with a HUGE skull over i kinda wonder where they got it from

the underworld is a city of ghouls, located in the Museum of History in the mall

Its a city of ghouls in the meuseum of history.

The skull is part of the museuam of history, if you talk to Carol she tells basically, the story of the nukes dropping, the aftermath, and the process of turning from human to ghoul. They also explain that the part of the museuam they live in was originally a tour of the "after" life, all kinda ironic.