What is Valuable?

Declaration of independence and the constitution?

Correct:

Two weeks after the Japanese attacked Pearl Harbor in 1941, the Declaration of Independence and the Constitution were sent by train to Fort Knox, Ky., for safekeeping. On the train trip, the compartments adjoining and connecting the sleeping car compartment containing the documents were occupied by armed Secret Service agents and Verner W. Clapp, who had recently been promoted to Chief Assistant Librarian of the Library of Congress.

Cool, I was kinda hoping my guess was wrong, and that it was something more mysterious.

It is interesting, though, that of all of the objects in the country, the items selected for this extreme level of protection were items of neither strategic nor financial value, of which thousands of copies existed.

It's not too surprising to me. The reverence toward those documents has always stood out to me (as a differentiating factor between the US and other nations I'd lived in).

This is interesting, what else they have stored there?

I think gold.

I found a little bit more:

On December 7, 1941, the Japanese attacked Pearl Harbor. On December 23, the Declaration and the Constitution were removed from the shrine and placed between two sheets of acid-free manila paper. The documents were then carefully wrapped in a container of all-rag neutral millboard and placed in a specially designed bronze container. It was late at night when the container was finally secured with padlocks on each side. Preparations were resumed on the day after Christmas, when the Attorney General ruled that the Librarian needed no "further authority from the Congress or the President" to take such action as he deemed necessary for the "proper protection and preservation" of the documents in his charge.

The packing process continued under constant armed guard. The container was finally sealed with lead and packed in a heavy box; the whole weighed some 150 pounds.

At about 5 p.m. the box, along with other boxes containing vital records, was loaded into an armed and escorted truck, taken to Union Station, and loaded into a compartment of the Pullman sleeper Eastlake. Armed Secret Service agents occupied the neighboring compartments. After departing from Washington at 6:30 p.m., the Declaration traveled to Louisville, KY, arriving at 10:30 a.m., December 27, 1941. More Secret Service agents and a cavalry troop of the 13th Armored Division met the train, convoyed its precious contents to the Bullion Depository at Fort Knox, and placed the Declaration in compartment 24 in the outer tier on the ground level.