Archives abroad: World War I-era documents declassified

The Central Intelligence Agency (CIA) has recently declassified six World War I-era documents — the oldest classified documents held by the spy agency dated 1917 and 1918 — containing information on the ingredients used to create invisible ink.

Currently held by the US National Archives they are believed to be the only remaining classified documents from the World War I era, according to the spy agency.

"These documents remained classified for nearly a century until recent advancements in technology made it possible to release them," CIA Director Leon Panetta said in a statement.

One of declassified documents, in particular, describes a number of recipes for making secret ink. You can also find detailed instructions on how to open a sealed letter without detection. Another memorandum dated June 14, 1918 — and written in French — reveals the formula used for German secret ink.