History books have recounted the tragic aerial assaults by the Imperial Japanese on Pearl Harbor on December 7, 1941. But discoveries made by scientists have revealed that the aerial assault was also accompanied by undersea submarine torpedo attacks – and one of them was at least partially successful in its mission. Beginning in 1993, two deep-diving submersibles operated by the Hawaii Undersea Research Laboratory (HURL), Pisces IV and Pisces V, embarked on a series of scientific and engineering dives in and around Pearl Harbor. They discovered what looked like a Type A Japanese mini-submarine, setting off a flurry of scientific activity and conjecture. After more than 15 years of subsequent research, PBS’s NOVA television series aired a documentary on January 5 confirming that this wreckage – found outside the harbor in 1,000 feet of water – is the fifth mini- or midget submarine that the Japanese Navy sent into the littoral waters of Pearl Harbor to wreak havoc on the U.S. Pacific Fleet that fateful day.

Four of these midget subs have been previously accounted for. They were all destroyed or run aground before becoming a threat in the attack. But the relatively recent discovery of this fifth submarine provided additional proof that underwater torpedoes were launched against several ships in the harbor, and one successfully penetrated the battleship USS Oklahoma. Admiral Chester Nimitz issued a report to Congress in 1942, describing an unexploded, 800-pound torpedo that was salvaged after the attack. The size of this torpedo was twice that of those carried by torpedo bombers, increasing speculation of submarine attacks. Furthermore, the remains of Oklahoma show damage that was caused by a torpedo much larger than that of an aerial torpedo.

What remains unknown is how this fifth submarine ended up in its current location — amidst other World War II debris in deeper waters outside the harbor. Theorists believe this fifth sub escaped the scene of the battle and was scuttled in a nearby area called the West Loch, but since the fate of the crew is unknown, this remains just a theory. A deadly ammunition explosion at the West Loch site in 1944 killed 200 sailors and wounded hundreds more. A subsequent salvage operation scooped up the remains of the exploded ship and deposited them offshore — in the same location where the fifth submarine was discovered. The state of the fifth submarine also reveals the evidence of a salvage operation; it is broken into 3-4 pieces, obviously separated for ease of transfer. Furthermore, there is steel cable attached that is consistent with salvage operations. But there is no record of its being salvaged, moved or deposited in its current location. As Dr. Robert Neyland, the Naval History and Heritage Command’s underwater archaeologist, said in a telephone interview last week, “None of the records as yet have spoken to this whole issue about a mini-sub being found in the West Loch area, salvaged and carried offshore.” The mystery continues…

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  • Chuck Hill

    The weight of the torpedo does not sound right?

  • CDR G (ret)

    You can take a look here:


  • D. E. Reddick


    The warhead weight would seem to be right. The Type 97 torpedo was a submarine-launched, 2161 lb., 17.7″ diameter weapon with a 772 lb. warhead. That warhead weight is close to the stated 800 lbs., if Adm. Nimitz’ report was in relation to the warhead size of the torpedo found. This particular torpedo type was operationally deployed and used once – during the attack on Pearl Harbor.

    “A miniaturized Type 93 intended for midget submarines. Very unsuccesful (its oxygen flasks leaked a lot), it was used operationally only once – at Pearl Harbor.” That’s from the Torpedoes page at the Combined Fleet site.

  • JV

    According to http://www.history.navy.mil/photos/events/wwii-pac/pearlhbr/ph-bba.htm, the Type 91 aerial torpedo warhead was approx 450lbs.

  • D. E. Reddick


    From the Torpedoes page at the Combined Fleet site we have this for the aircraft-launched Type 91 Mod 2 torpedo type used by the IJNS during the attack on Pearl Harbor. Its warhead was about 58 percent the size of the midget submarine launched Type 97 torpedo (452 vs. 772 lbs.). The described damage to USS Oklahoma (comparatively speaking) and the finding of the ‘800-pound torpedo’ (772 lb. warhead) would have matched USN expectations when compared to the 825 lb. warhead of the then current USN Mark 15 torpedo (the torpedo type with the egregiously defective detonator which cursed USN combat efforts during the early years of WW-II).

    Type 91 Mod 2: 17.7″ diameter; 18′ 0″ length; 1841 lbs. weight, overall; 452 lbs. weight, warhead; 2000m @ 41 knots range & speed; 260 knots max. launch speed.

    “First deployed in April, 1941. Carried by the ‘Kate’ torpedo bombers which attacked Pearl Harbor.”

  • Chuck Hill

    Thanks, that makes sense now.