Researchers in Hawaii say they've mapped and photographed the wreck of a World War II-era Japanese "megasubmarine," so big it included a hangar from which it could launch float-plane bombers.
The I-400 model submarine of the Imperial Japanese Navy was first discovered in December 2013 by a team of researchers from the University of Hawaii and the U.S. National Oceanic and Atmospheric Administration.
Researchers have returned to the wreck off the coast of Oahu as part of a project by NHK, the Japanese Broadcasting Corp., to locate key pieces of the I-400 for a television documentary scheduled to be broadcast in May.
The recent dives allowed researchers to map the wreck in detail and capture video of the submarine's aircraft hangar, which could hold three aircraft that could be launched when the sub surfaced, and of its conning tower.
The sub, captured near the end of the war, was deliberately sunk by the U.S. Navy in the waters off Pearl Harbor to keep its design innovations and technology a secret from the Soviet Union.
Its exact location had been lost until it was rediscovered in the 2013 search in 2,300 feet of water.
The I-400 had "been on our 'to-find' list for some time," Terry Kerby, Hawaii Undersea Research Laboratory operations director and chief submarine pilot, said at the time. "It was the first of its kind of only three built, so it is a unique and very historic submarine."
The new expedition was mounted to gather better information on the sub and its wreckage, he says.
"We didn't have detailed enough bottom mapping data to help locate the hangar, conning tower, and other signature features missing from the wreck of the I-400," he explains. "With only one dive day to try to find anything, we knew there was a strong chance we might spend the dive looking at the barren sandy bottom."
Approaching the wreck of the main hull, divers found the hangar, conning tower and the submarine's bell, torn off by the explosions when the I-400 was scuttled and sunk.
At 400 feet in length -- longer than a football field -- the I-400 was the biggest submarine ever built until the advent of nuclear-powered subs in the 1960s.
Its outstanding feature was its deck-mounted aircraft hangar, from which three folding-wing floatplanes could be launched by catapult within minutes of the sub's surfacing.
The submarine had sufficient range to approach the coast of the U.S. and launch its planes on bombing missions, but no such missions were carried out before the war came to an end.
"The innovation of air strike capability from long-range submarines represented a tactical change in submarine doctrine," says James Delgado, director of NOAA's Maritime Heritage Program.
"Following World War II, submarine experimentation and design changes would continue in this direction, eventually leading to ballistic missile launching capabilities for U.S. submarines at the advent of the nuclear era," he says.
