This page discusses the Strategic Stockpile of silver in the United States. For some reason, there is very little information available about it.
The United States keeps stockpiles of strategic materials. It is run by the "DLA (Defense Logistics Agency) Strategic Materials." Before July, 2010 it was run by the Defense National Stockpile Center; the stockpile itself is called the United States National Defense Stockpile. It was created after World War II to store critical strategic materials of national defense purposes. This helps ensure that the United States will have the materials it needs in the time of a crisis. You can also read an overview of the agency.
The Silver Strategic Stockpile (then called the Strategic and Critical Materials Stockpile) was formed in June, 1968, with a 'donation' of 165 million ounces of silver from the U.S. Treasury. 25.5 million ounces was removed in 1970, leaving 139.5 million ounces. It stayed at about that level through 1985, after which it declined each year until 2002 when it was all used up (the silver was sold to the Treasury for making Silver Eagles). As early as 1979 it was determined that the silver was not necessary, because "the probable wartime supply exceeds projected U.S. requirements" (Report By The Comptroller of the United States, 'National Defense Requirements For A Silver Stockpile', April 10, 1979). The silver was stored in West Point (49.4Moz) and San Francisco (90.1Moz) (same source).
To see the exact levels per year, you can look at our Silver Stockpiles of the Past page.