Do you want to read the oldest classified documents in our collection? Learn how to make invisible ink? See the only classified documents still in existence from the first World War? Then you’re in luck.

Five years ago, the CIA declassified the US Government’s six oldest classified documents, dating from 1917 and 1918. These documents, which describe secret writing techniques and are housed at the National Archives, are believed to be the only remaining classified documents from the World War I era. Documents describing secret writing fall under the CIA’s purview to declassify.

“These documents remained classified for nearly a century until recent advancements in technology made it possible to release them,” former CIA Director Leon Panetta said during the document’s release 2011. “When historical information is no longer sensitive, we take seriously our responsibility to share it with the American people.”

One document outlines the chemicals and techniques necessary for developing certain types of secret writing ink and a method for opening sealed letters without detection. Another memorandum dated June 14, 1918 – written in French – reveals the formula the German’s used to produce invisible ink.