Sagami Naval Arsenal Ruins
相模海軍工廠跡
Ruins of the Imperial Japanese Navy's dedicated chemical weapons and pyrotechnics arsenal, established in May 1943 in Samukawa Town, Kanagawa Prefecture. The facility was promoted from the Chemical Research Division of the Naval Technical Research Institute (Hiratsuka) to enable mass production of mustard gas (yperite) bombs, gas masks, and incendiary ordnance. At peak capacity over 3,500 workers — including conscripted laborers, Women's Volunteer Corps members, and mobilized students — operated within the 700,000 m² compound. In 2002, buried mustard gas bottles were discovered at the former site during expressway construction, injuring 11 workers and confirming the enduring contamination legacy.
Images
History
The Sagami Naval Arsenal (相模海軍工廠, Sagami Kaigun Kōshō) was an Imperial Japanese Navy facility in Samukawa Town (寒川町), Koza District, Kanagawa Prefecture, dedicated to the research, development, and mass production of chemical weapons and pyrotechnic ordnance. It was one of the IJN's most secretive wartime installations.
Origins and Establishment
Japan's naval chemical weapons program traces its roots to the aftermath of World War I. In 1922, the Naval Ordnance Headquarters (艦政本部) established a department for chemical warfare research. The following year, in April 1923, this function was absorbed into the newly formed Naval Technical Research Institute (海軍技術研究所) in Hiratsuka, Kanagawa Prefecture, where a dedicated Chemical Research Division (化学研究部) conducted research on toxic agents, protective equipment, and incendiary devices throughout the 1920s and 1930s.
As the Pacific War intensified and demand for chemical munitions surged, the Navy determined that the Hiratsuka research facility was insufficient for mass production. In May 1943 (Showa 18), the Chemical Research Division was upgraded to full arsenal status, and the Sagami Naval Arsenal was formally established on a 700,000 m² (approximately 173-acre) site in Ichinomiya, Samukawa Town. The Hiratsuka facility continued to operate as the arsenal's Chemical Experimental Division (化学実験部).
Production and Operations
The arsenal's primary products included:
- Type 6 No. 1 Land-use Bombs (六号一型陸用爆弾) — 60 kg bombs filled with 17 kg of yperite (mustard gas), the arsenal's signature weapon
- Other chemical munitions containing blister agents (mustard gas, lewisite) and tear agents (chloroacetophenone)
- Gas masks and chemical protective equipment — production of defensive gear was actually the facility's largest output by volume, reflecting the Navy's emphasis on chemical defense over offense
- Pyrotechnic and incendiary ordnance (火工兵器) including signal flares, tracer ammunition components, and incendiary devices
The workforce at peak capacity comprised over 3,500 personnel: naval officers, civilian employees, conscripted workers (徴用工), Women's Volunteer Corps members (女子挺身隊), and mobilized students (勤労動員学徒). The arsenal was served by the Nishi-Samukawa Branch Line (西寒川支線) of the Japanese National Railways Sagami Line, which provided rail access for materials and finished ordnance.
Wartime Injuries and Secrecy
Workers at the arsenal suffered chemical exposure injuries during production, particularly from mustard gas handling. The extreme secrecy surrounding chemical weapons production meant that many workers did not fully understand the hazardous nature of the materials they handled. Some survivors continued to suffer from chronic health effects for decades after the war, including skin conditions and respiratory problems.
Postwar Legacy
After Japan's surrender in August 1945, the arsenal was dismantled. Much of the site was converted to civilian industrial and residential use. The Nishi-Samukawa Branch Line continued to operate until its closure in 1984, and the former station site was converted into Hakkaku Plaza (八角広場), a small park featuring a memorial monument with preserved railroad ties and rails commemorating the branch line and the arsenal's history.
The arsenal's most dramatic postwar episode occurred on September 25, 2002, when construction workers building the Ken-Ō Expressway (圏央道) overpass on the former arsenal grounds unearthed beer bottles containing mustard gas and other chemical agents. Eleven workers were injured, developing rashes and blisters from exposure. Subsequent excavation revealed over 800 suspicious bottles — 382 containing mustard gas and 237 containing lewisite or chloroacetophenone — along with approximately 7,000 m³ of contaminated soil. The Japanese Ministry of Defense confirmed the materials as Imperial Navy chemical weapons. The site remains designated as an "A-Case Zone" (A事案区域) by the Japanese government, indicating confirmed presence of deteriorated chemical weapons, and all excavation activities in the area require specialized safety protocols.
The former Sagami Naval Arsenal site stands as a sobering reminder of Japan's wartime chemical weapons program and the enduring environmental consequences of military-industrial contamination.