Iwahana Explosives Factory Ruins (Gunma-no-Mori Park)

The Iwahana Explosives Manufacturing Plant (岩鼻火薬製造所, Iwahana Kayaku Seizōsho) was established in November 1882 (Meiji 15) as the Tokyo Artillery Arsenal Iwahana Explosives Manufacturing Plant (東京砲兵工廠岩鼻火薬製造所). It was Japan's second army explosives facility after the Itabashi plant in Tokyo. The site in Iwahana village (now Iwahana-chō, Takasaki) was selected for its favorable location along the Karasu and Ino rivers, which provided water transportation to Tokyo and water-wheel power for the manufacturing process.

The facility initially produced black powder (kokushoku kayaku). In 1905, during the Russo-Japanese War, the Army's successful use of dynamite in the siege of Port Arthur's Higashi-Keikanzan North Fort prompted the urgent construction of a dynamite factory within the Iwahana compound. Under the direction of Dr. Ishifuji Toyota, Japan's first domestic dynamite plant was completed, and production of kieselguhr dynamite and gelatin dynamite began in 1906 (Meiji 39). This earned the site the distinction as "the Birthplace of Dynamite in Japan" (我が国ダイナマイト発祥の地).

After the Itabashi plant ceased black-powder production in 1906 and the Meguro plant closed in 1924, Iwahana became the Army's sole black-powder manufacturer. From 1934, the facility also began producing smokeless powder (G-type/無煙火薬). The plant underwent several administrative reorganizations: renamed to Army Ordnance Fire Engineering Arsenal Iwahana Explosives Manufacturing Plant (陸軍造兵廠火工廠岩鼻火薬製造所) in April 1923, and then to Tokyo Second Army Ordnance Arsenal Iwahana Manufacturing Plant (東京第二陸軍造兵廠岩鼻製造所) in April 1940.

A dedicated narrow-gauge railway, the Iwahana Light Railway (岩鼻軽便鉄道), was completed in 1917 primarily for transporting military supplies, with tracks running through the facility grounds to connect with the national rail network.

The most devastating accident in the plant's history occurred on December 19, 1938. Amid the rapid production increases driven by wartime mobilization following the outbreak of the Second Sino-Japanese War, four separate explosions struck the facility that year alone. The December incident — a black-powder explosion at 2:35 PM — triggered secondary detonations of approximately 4,800 kilograms of stored powder across three locations, killing 13 workers, injuring 5, and damaging 560 civilian homes in the surrounding area. In total, 24 workers died in the four 1938 explosions.

By August 1945 at the time of Japan's surrender, the facility occupied approximately 325,000 tsubo (roughly 1,072,500 square meters / 265 acres), operated 4,000 major machines, and employed 3,956 workers. Monthly production capacity stood at 110 tons of smokeless powder, 32 tons of black powder, and 180 tons of dynamite — the highest output figures in the plant's 63-year history.

After Japan's surrender, the facility was closed and the vast grounds were partitioned. The Japan Atomic Energy Agency (originally Japan Atomic Energy Research Institute) established its Takasaki Advanced Radiation Research Institute on part of the site. The chemical manufacturer Nippon Kayaku Co., Ltd. (日本化薬) took over another section for its Takasaki plant. The remaining central portion between these two entities was designated for Gunma Prefecture's Meiji Centennial Memorial Project (明治百年記念事業) and opened in 1974 as Gunma-no-Mori Park (群馬の森), a 26.2-hectare prefectural park.

Today the park houses the Gunma Prefectural Museum of History (群馬県立歴史博物館, opened 1979) and the Gunma Prefectural Museum of Modern Art (群馬県立近代美術館). Scattered throughout the park grounds are tangible remnants of the explosives factory: massive earthen blast-protection embankments (dorei/土塁) originally built to contain explosion damage from individual powder-processing workshops; concrete-lined tunnels that served as protected passages into the workshops; ruins of ammunition magazines (danyakuko/弾薬庫); boundary markers inscribed "Army Land" (陸軍用地); and a large earthen embankment that formed the backstop of the factory's weapons testing range (shajō/射場), stretching roughly 30–40 meters long and 4 meters high, though this area is fenced off.

The "Birthplace of Dynamite in Japan" memorial stone (我が国ダイナマイト発祥の地碑) was erected in 1973 on the park's central lawn. Its inscription records the 64 years (1882–1945) of explosives production and acknowledges the facility's significant role in Japan's modern industrial history, supplying both military and civilian needs. The site stands as a rare example of a major Imperial Army munitions complex whose physical traces remain accessible within a public park, offering visitors a direct encounter with Japan's wartime industrial heritage.