The Imperial Japanese Navy engaged in significant espionage activity leading up to the infamous surprise attack on Pearl Harbor, December 7, 1941. In addition to providing the midget submariners with Lists of Safe Contacts with whom they might seek shelter, they also provided a detailed map of the Harbor and how the U. S. Pacific Fleet might be anchored. The midget submariners were also given crash courses in English as it was not the intention of Admiral Yamamoto that the midget submariners needlessly sacrifice their lives. He fully expected at least some of them to return for debriefing and congratulatory celebrations.
Details of the harbor were obtained by a combination of sight-seeing fly-overs in civilian planes, ground observation and casual interviews with U. S. personnel. This is a copy of the map Masaharu Yokoyama was using on December 7, 1941.