How do you get into the archival strongroom in Fallout 3-naninal-archives?

Answer 1

The National Archives are in the Mall next to the Musem of History, if you start the quest "Stealing Independence" you are asked to find the decleration of independance it will show you where it is, you can find Abraham Washington in Rivet City in the National Preservation Society.

Answer 2

Before the strongroom door is even available to be opened (the yellow light will indicate that it is available) you need to first locate the 5 Keller holotapes and listen to them to know the passcode.

Answer 3

Inside the main rotunda there will be a woman named Sydney who after you help her survive the super mutant onslaught she will talk to you about the Declaration of Independence. After this she will activate a terminal which opens a secret elevator into the strongroom of whcih after you kill the robots you can find the Declaration of Independence. If you accidentely kill Sydney go to where you first entered the building and look along the left wall for a door or corridor. Once you find this, follow it and you will eventually reach the strongroom.