Having ridden the backs of fleet submarines from Japan, five midget subs slipped free of their mother ships in the first hours of December 7, 1941. Their orders: Swim the ten or so miles (16 kilometers) to Pearl Harbor, wait for the air attack to begin, make as much trouble as you can, sneak back out to sea, and rendezvous with your fleet sub.

None of the mini-subs ever saw their mother ships again. One ran aground and was captured, another is unaccounted for, another was sunk in the harbor, and two were sunk off the harbor entrance. Of the three confirmed sunk, two have been raised. The other was the first “kill” of the battle that thrust the U.S. into World War II—and is this expedition’s target.

Like her sisters, the sub we’re looking for (her name is unknown) held two sailors and two torpedoes, was battery powered, and measured 79 feet (24 meters) long and about 6 feet (1.8 meters) in diameter. She met her end at 6:45 a.m. local time, in restricted waters about a mile (1.6 kilometers) off Oahu. There the destroyer Ward sent her sinking some 1,200 feet (366 meters) to the seafloor.

U.S. forces failed to recognize the sub for the omen it was. At 7:49 a.m. 183 Japanese planes, just over half of the air- attack force, caught the island unawares.

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Ha-19, a Japanese midget submarine that ran aground on Oahu after the Pearl Harbor attack and is nearly identical to the sub Ballard is seeking Photograph courtesy U.S. Naval Historical Center (photo#:NH91333)