Sunday’s Honolulu Star-Bulletin reports on a new undersea discovery in waters off Hawai‘i.
During test dives Thursday, the Hawaii Undersea Research Laboratory’s Pisces submarines found the remains of the Imperial Japanese Navy’s I-401 submarine, a gigantic underwater aircraft carrier built to bomb the Panama Canal.
“We thought it was rocks at first, it was so huge,” said Pisces pilot Terry Kerby. “But the sides of it kept going up and up and up, three and four stories tall. It’s a leviathan down there, a monster.”
It is not the first World War II-era “monster” that the HURL scientists have found. Last year, off Pearl Harbor, they located the wreck of the gigantic seaplane Marshall Mars, one of the largest aircraft built and used as a transport plane by the U.S. Navy. Two years earlier in the same area, the HURL crew also found the wreckage of a Japanese midget sub that was sunk on Dec. 7, 1941.
The latest HURL discovery is from the I-400 “Sensuikan Toku” class of submarines, the largest built prior to the nuclear ballistic missile submarines of the 1960s. They were 400 feet long and 39.3 feet high, could reach a maximum depth of 330 feet, and carry a crew of 144.
Each carried three fold-up bombers inside a watertight hangar, plus parts to construct a fourth airplane. The bombers, called Seiran or “Mountain Haze,” [but see note below] 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.
For much, much more on the sub’s mission, see this site.
NOTE: The Combined Fleet website consistently translates Seiran (晴嵐) as ‘Mountain Haze’, ignoring the meaning (‘clear, not cloudy’) of the first character, but staying truer to the usual Chinese meaning of the second character. However, the Smithsonian’s National Aerospace Museum’s website translates it as ‘Clear Sky Storm‘, which captures the most common Japanese senses of both characters (hare ‘clear skies’ and arashi ‘storm’) and strikes me as a more natural name for a surprise attack submarine-carried bomber, perhaps implying something like ‘Bolt from the Blue’.
The Funatsu Aviation Instrument Museum’s website gives a guide to the naming conventions of Japanese naval warplanes. Fighters were named after strong weather (-fuu/-puu ‘-wind’, -rai ‘-thunder’); (high-altitude) bombers after constellations (-sei ‘-star’); reconnaissance planes after clouds (-un); attack (dive?) bombers after mountains (-san/-zan); patrol planes after seas (-kai); transports after skies (-kuu); trainers after plants (-giku ‘chrysanthemum’, -gusa ‘grass’), and others after scenery. This is helpful except that Seiran appears under the Other category in one listing, and under the Attack Bomber category elsewhere.
Far Outliers 
