According to the "Nanjung Ilgi", Yi's wartime diary, Yi decided to resurrect the turtle ship in 1591 from pre-existing designs (see picture, illustrated nearly 200 years later) after discussing the matter with his subordinates. Once concluding that a Japanese invasion was possible, if not imminent, Yi and his subordinate officers, among whom Na Dae-yong (羅大用) is named as the chief constructor, designed and built the first modern turtle ship. Yi's diary, along with the book entitled "Hangnok" written by his nephew Yi Beon, described numerous important details about the structures, construction progress, and the use of turtle ships in battle, as well as the testing of weaponry used in the ships.