Peenie Wallie

Each carried three fold-up bombers inside a watertight hangar, plus parts to construct a fourth airplane. The bombers, called Seiran or "Mountain Haze," could be made ready to fly in a few minutes and had wing floats for return landings. Fully loaded with fuel, the submarines could sail 37,000 miles, one and a half times around the world. Three were captured at the end of the war, as well as a slightly smaller test design called the I-14.

Their first mission was called "Operation PX," a plan to use the aircraft to drop infected rats and insects with bubonic plague, cholera, dengue fever, typhus and other diseases on American West Coast cities. When the bacteriological bombs could not be prepared in time, the target was changed to the Panama Canal.

I-400 and I-401 were captured at sea a week after the Japanese surrendered in 1945. The commander committed suicide and the huge submarines' mission was never completed.

I-400, I-401 and I-14 were ordered to sail to Pearl Harbor in late 1945 with an American prize crew, who smuggled Japanese war souvenirs in the aircraft hangars. Also along to be evaluated were I-201 and I-203, two top-secret Imperial Navy submarines that were twice as fast as American designs.

The submarines were greeted with ceremonial brass bands in early 1946, but within a few months it was decided to scuttle the Japanese designs, partly because Russians scientists were demanding access to them. On May 31, 1946, I-401 and the other four top-secret Japanese submarines were sunk by torpedoes from the American submarine USS Cabezon. I-401 was last seen sinking by the stern, vanishing until last week.

"The main hull is sitting upright on the bottom, and it's in great shape," said Kerby. "The I-401 numbers are clearly visible on the sides of the conning tower, and the antiaircraft guns are in almost perfect condition."