In April 2004, 40 miles off the coast of Nagasaki, Japan, ProMare, in partnership with Discovery Channel film-makers, located 24 World War II Japanese submarines.  The secret site was suspected to be the location of the subs, but remained unexplored, until this expedition. As part of a Discovery Channel documentary, Sen Toku: The Search for Japan’s Secret Subs, the site was documented and its existence verified. This find represented the single largest known underwater collection of WW II submarines, and the expedition marked the first major marine exploration undertaken in Japan.

The underwater expedition employed state-of-the-art ROVs (remotely operated vehicles), to descend 600 feet and record, for the first time on film, the I-58, responsible for one of the worst disasters in US naval history, when it sunk the USS Indianapolis on 30th July 1945.  The expedition also revealed the site to be the resting place of the I-402, or Sen Toku (Japanese for “Secret Sub”) as well as 22 other remarkable submarines, ranging in size from 45 to 122 meters. Brett Phaneuf, director ProMare made the discovery, accompanied by Bob Asplin of Kongsberg Simrad (Vancouver BC). Ian Herring of Parallax Films produced the documentary.

The largest submarine built during World War II (and remaining the world’s largest until the 1960s), the Sen Toku was an engineering marvel and the pinnacle of WWII underwater technology. Its enormous size and attack capability—it was also an underwater aircraft carrier—made it a threat to Japan’s enemies. But, the I-402 never fought a battle.