The I-400, a huge Imperial Japanese Navy submarine that was lost during World War II has been found at a depth of 2,300 feet of water off the southwest coast of O'ahu.

The submarine, larger than a football field, belonged to the "Sen-Toku" class submarines, which were the largest of its kind made at the time. The I- 400 and its sister ship- the I 401- could travel one and half times around the world without refuelling.

The mammoth submarine was discovered by Terry Kerby at the HawaiÊ»i Undersea Research Laboratory (HURL) and colleagues. The HURL team used side-scan sonar and multi-beam sonar data to detect any wreckage on the seafloor. Researchers confirmed that the debris was of the mega-sub after analyzing its aircraft launch ramp, stern lighting and deck crane. The sub's aircraft hangar was found to be separated from the remaining body, possibly due to U.S. Navy torpedo blasts that sunk the monster in 1946.    

                                     

"The I-400 has been on our 'to-find' list for some time. It was the first of its kind of only three built, so it is a unique and very historic submarine," said Kerby in a news release. "Finding it where we did was totally unexpected. All our research pointed to it being further out to sea. The multi-beam anomalies that appear on a bottom survey chart can be anything from wrecks to rocks-you don't know until you go there." Since 1992, HURL has used its manned subs Pisces IV and Pisces V to look for wrecks on the ocean floor, according to a news release.

The 400-feet aircraft-carrying submarines could hold three folding-wing M6A1 Seiran bombers, The New York Times reported. These planes could be launched in minutes after the submarines surfaced. Each aircraft could carry about 1,800 pounds of explosives.

"The innovation of air strike capability from long-range submarines represented a tactical change in submarine doctrine," said Drs. James Delgado, director of NOAA's Maritime Heritage Program, within the Office of National Marine Sanctuaries, Washington, D.C. "The large I-400, with its extended range and ability to launch three M6A1 Seiran strike aircraft, was clearly an important step in the evolution of submarine design."

The U.S Navy captured five Japanese subs, including the behemoth I-400 by the end of the war and brought it to Pearl Harbour for inspection. However, it hurriedly sank the subs off the coast of Oʻahu after the Soviet Union demanded access to the submarines, in fear that the technology would fall into wrong hands.