Mitsubishi Arms Sumiyoshi Tunnel Factory Ruins

The Mitsubishi Arms Sumiyoshi Tunnel Factory (三菱兵器住吉トンネル工場) was an underground dispersal factory constructed during the final year of the Pacific War to protect torpedo production from increasingly devastating Allied air raids. Built as an evacuation facility for the Mitsubishi Heavy Industries Nagasaki Arms Factory (三菱重工業長崎兵器製作所), the tunnels were carved into a hillside between the Sumiyoshi and Akasako neighborhoods in the northern part of Nagasaki City.

Construction began in August 1944 using conscripted Korean laborers, estimated at 800 to 1,000 workers. Six parallel tunnels were excavated at roughly ten-meter intervals, each approximately 300 meters long, 4.5 meters wide, and 3 to 4 meters high — large enough to house fully equipped manufacturing lines. Tunnels No. 1 and No. 2 were completed by April 1945 and immediately put into production, while tunnels No. 3 and No. 4 were finished but never fully operational, and tunnels No. 5 and No. 6 remained under construction at war's end. The factory operated around the clock in two or three shifts, employing approximately 1,800 workers including Mitsubishi employees, mobilized students, and members of the Women's Volunteer Corps, who manufactured components for the Type 91 aerial torpedo.

On August 9, 1945, at 11:02 a.m., the United States dropped the Fat Man plutonium bomb over Nagasaki. The Sumiyoshi tunnels were located approximately 2.3 kilometers north of the hypocenter. While workers outside the tunnels were killed or severely burned by the blast and thermal radiation, those inside the tunnels survived with relatively minor injuries — about 20 workers were hurt by falling rocks. In the immediate aftermath, the intact tunnels became a critical emergency shelter and first-aid station. Survivors from the devastated Mitsubishi Arms Ohashi Factory (大橋工場), which had been located much closer to the hypocenter, streamed into the tunnels seeking refuge. Workers who had been inside organized rescue parties and rendered first aid to the flood of injured civilians.

After Japan's surrender, the tunnels were abandoned and fell into disuse for decades. In 2010, Nagasaki City completed restoration work on tunnels No. 1 and No. 2 on the Sumiyoshi side, opening them to the public on March 30, 2010. Visitors can normally view the tunnel interiors through barriers at the entrance, with full interior access permitted during the Peace Memorial period (August 7–10) and UN Disarmament Week (October 24–30). A recovered Type 91 torpedo has been on display at the site since September 2012, providing a tangible connection to the factory's wartime production. The site is designated as a Class B atomic bomb-related structure by Nagasaki City and serves as an important peace education resource.