It goes underneath the Kanmon Straits, connecting the islands of Honshu and Kyushu.

Its construction began in 1936, and it was completed in November 1942, during the Pacific War.

Near the end of the Pacific War the Allies planned to blow up the two tunnels with 50,000 pounds of explosives as part of the invasion of Japan.

The Office of Strategic Services trained 250 of its agents for the task, but the surrender of Japan occurred before they were needed.

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