The Mariana Islands, an archipelago of 15 volcanic islands located southwest of the Hawaiian Islands and northeast of the Philippines, were strategically situated to serve as an air base for the Army Air Corps’ long-range bombers to make non-stop strikes on nearby Japan. Because of this, American forces constructed air bases on the islands of Guam and Saipan. Japan saw the Marianas as the last line of defense for the protection of Japan and its possessions in the south Pacific.
On March 31, 1944, two planes bound for Mindanao were caught up in a tropical storm. One plane carried Admiral Mineichi Koga, who replaced Admiral Isoroku Yamamoto as commander-in-chief of the Japanese Combined Fleet after Yamamoto’s death. The other carried Vice Admiral Shigeru Fukudome. Both crashed into the sea off the southern Philippines. When Koga’s plane smashed into the ocean, it left no survivors. Fukudome and ten of his staff, however, survived their plane crash.
These survivors were picked up by Filipino guerrillas, but left behind was a box containing a leather-bound copy of documents bearing the letter “Z” on its red cover, as well as other documents, including a study of fleet operations and a sheet of code rules. When it later washed ashore, it was recovered by two Filipino men who, guessing at their value, quickly dried the documents and hid them, buried underground in the box.