You mean the Museum of History and the Ghouls have made it Underworld but I think it is where some slavers are around the side of the building behind the Washington Monument (Don't know much American History :P ) and the slavers attack if you go past them so go around to a little entrance just next to the steps on the right and you can find a memorial place with it in there, but 1 slaver will attack you as they are the only friendly slaver unless you steal something and get caught!