1 / 20 2 / 20 3 / 20 4 / 20 5 / 20 6 / 20 7 / 20 8 / 20 9 / 20 10 / 20 11 / 20 12 / 20 13 / 20 14 / 20 15 / 20 16 / 20 17 / 20 18 / 20 19 / 20 20 / 20 ❮ ❯ 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.