NOVA's team of expert investigators journey to the seafloor to explore the wreckage of the most mysterious of these subs. Did this mini-sub and its two-person crew make it into Pearl Harbor and fire torpedoes at the Arizona ? They pursue this puzzle with unprecedented access to the remains of the Arizona and other unique evidence, including aerial photos taken by Japanese aircraft and moving testimonials from . and Japanese veterans. "Killer Subs in Pearl Harbor" is a gripping investigation of the possibility that these tiny but lethal mini-subs may have played a crucial and previously unsuspected part in the tragic events of that "Day of Infamy."
When a new edition was released, the cryptographers were forced to start again. The original JN-25A system replaced the 'Blue' code (as Americans called it), and used five-digit numbers, each divisible by three (and so usable as a quick, and somewhat reliable, error check, as well as something of a 'crib' to cryptanalysts), giving a total of 33,334 legal code values. To make it harder to crack a code value, meaningless additives (from a large table or book of five-digit numbers) were added arithmetically to each five-digit cipher element. JN-25B superseded the first release of JN-25 at the start of December 1940. JN-25B had 55,000 valid words, and while it initially used the same additive list, this was soon changed and the cryptanalysts found themselves entirely locked out again.
Striking the Pacific Fleet at anchor in Pearl Harbor carried two distinct disadvantages: the targeted ships would be in very shallow water, so it would be relatively easy to salvage and possibly repair them; and most of the crews would survive the attack, since many would be on shore leave or would be rescued from the harbor. A further important disadvantage—this of timing, and known to the Japanese—was the absence from Pearl Harbor of all three of the . Pacific Fleet's aircraft carriers ( Enterprise , Lexington , and Saratoga ). IJN top command was so imbued with Admiral Mahan 's " Decisive battle " doctrine—especially that of destroying the maximum number of battleships—that, despite these concerns, Yamamoto decided to press ahead.  [ page needed ]