Photo/Illutration The interior of a tunnel of the Hamabira underground bunker (Provided by the Tarumizu City Board of Education)

TARUMIZU, Kagoshima Prefecture--A sprawling tunnel system discovered in a crumbling cliff face here almost certainly dates to the wartime era, say local education officials who believe it was used by the Imperial Japanese Navy to maintain torpedoes.

The 1.75 kilometers of tunnels make the complex “one of the largest underground bunkers in the Kyushu region” from the waning days of World War II, an official said.

The T-shaped system in the city’s Hamabira section was discovered during survey work in April 2022 to stabilize a cliff that had collapsed. It was built beneath volcanic rock that stands 40 meters above ground level facing Kagoshima Bay.

The network, now called the Hamabira special underground bunker, has 11 straight tunnels running parallel to each other, according to the Tarumizu City Board of Education.

Of these, 10 stretch for 100 meters. The remaining tunnel is only about 50 meters long, suggesting it was not completed before the war ended.

The tunnels link to one that is 170 meters long and built at a right angle in the back of the bunker system with smaller access tunnels in between.

Overall, the tunnels are up to 4 meters in height and width.

The board concluded the tunnel system, which covers an area of 5,138 square meters, is part of the Tarumizu Naval Air Corps complex, citing its “extremely sturdy structure, different from air-raid shelters for citizens” and records on the local history.

The bunker is located near the Tarumizu Naval Air Corps base, which was in charge of training student operators to do maintenance for aerial torpedoes during the war.

Founded in February 1944, the naval air corps trained about 2,500 student operators at its peak.

The bunker is thought to have been constructed around March 1945, less than six months before Japan’s surrender.

A few discoveries during the board’s investigation point to a connection to torpedoes.

They included hulks of three presumed fuel storage tanks to power a diesel generator that was situated nearby. 

There were also hook-shaped depressions on the inner walls where heavy torpedoes were believed to have been suspended, board officials said.

Most of the discoveries were concentrated in the northern section of the bunker.

“That indicates construction began in the northern section, but Japan lost the war before the work in the southern section could be finished,” an education official said.

The bunker was built into sturdy rock, making preservation of the historic site easier.

“It was possible to build multiple 100-meter straight tunnels because the site was in an area created from layers of a pyroclastic flow,” the official said.

The bunker was declared off-limits after work began to stabilize the cliff.

The city board plans to show 3D footage of the interior of the complex at a later date.

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