I-400 Sub
HURL discovered the I-400 sub wreck off the coast of Oahu. NOAA/HURL

There's now one less item on the Hawaii Undersea Research Laboratory's "to find" list, following the discovery of the historic Japanese World War II submarine I-400. According to HURL, the 400-foot sub was the largest ever built prior to the construction of nuclear-powered subs and had a range of 37,500 miles.

According to Reuters, the sub was set to attack the Panama Canal before it was sunk by U.S. forces. The I-401, its sister ship, was found off the coast of Oahu in 2005. In an HURL press release, the group said the I-400 Sen-Toku class sub was an impressive feat of engineering then and remains so to this day. It could travel around the world one and a half times before refueling, something that has yet to be replicated by other diesel-electric subs. Both the I-400 and the I-401 were large enough to hold three folding-wing bombers (Aichi M6A1 Seiran).

Terry Kerby, HURL operations director and chief submarine pilot, led the team that discovered the sub, which had been sunk by the United States after it was captured and has been lying off the coast of Oahu since 1946. Kerby was researching wrecks off the coast of Hawaii, and Oahu in particular, and was hoping to find the I-400 wreck. With the help of archaeologists from the National Oceanic and Atmospheric Administration (NOAA) familiar with WWII wrecks, Kerby used HURL’s manned submersibles, named Pisces IV and Pisces V, to dive more than 2,300 feet and discover the remains of the I-400 sub.

“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 statement. He said that the team expected to discover the sub further out than they did but searched the area after sonar readings indicated a large wreck.

At the end of WWII, the U.S. captured five Japanese submarines, of which HURL has found four off the coast of Oahu, with one more Sen-Toku class sub to be discovered. The U.S. denied any knowledge of where the subs were sunk during the Cold War, reports Reuters. The I-400 wreck was discovered in August 2013 and confirmed after HURL’s findings were reviewed by the U.S. State Department and the Japanese government.

Video of the I-400 discovery can be viewed below.