Usa Naval Air Base Ruins

Usa Naval Air Base (宇佐海軍航空隊) was established on October 1, 1939, on the Usa Plain in what is now Usa City, Oita Prefecture. Covering approximately 184 hectares with around 800 personnel, it served as a training facility for Imperial Japanese Navy carrier-based attack aircraft (kanko) and dive-bomber (kanbaku) crews destined for front-line carrier air groups.

With the outbreak of the Pacific War in December 1941, the base was progressively drawn into operational roles. By 1944, as Japan's strategic position deteriorated, construction began on a network of hardened aircraft shelters — the distinctive concrete revetments (掩体壕, entaigo) that survive today. Workers, including mobilized students and local civilians, built at least 10 covered reinforced-concrete shelters and numerous open earthen revetments across the surrounding farmland to disperse and protect aircraft from Allied air attacks.

In early 1945, the base became a staging point for tokko (special attack / kamikaze) operations. The Hachiman Gokō-tai special attack units formed at Usa advanced to Kushira airfield in Kagoshima and sortied on April 6, 12 and 16, 1945 (96 aircraft in the three waves); 81 planes and 154 young airmen were lost. The youngest were barely out of their teens. A memorial stone on the northeast side of the Jōi No. 1 Revetment honors their sacrifice.

From spring 1945, U.S. carrier aircraft and B-29 bombers subjected Usa to repeated devastating raids. The worst came on April 21, 1945, when over 136 tons of ordnance were dropped in approximately 11 minutes, killing more than 320 base personnel and destroying most facilities. The bombing created massive craters, one of which survives as Bakudan-ike (Bomb Crater Lake) with a viewing platform. Surrounding communities — schools, temples, homes — also suffered heavily, with bullet-scarred walls still visible at Yanagigaura Elementary School and the Renkō-ji temple gate.

The air group was officially disbanded on May 5, 1945, though raids continued until August 8, just one week before surrender.

Today the Usa war ruins constitute the most extensive surviving WWII air base archaeological landscape in Kyushu. Key preserved structures include:

• Jōi No. 1 Revetment (城井1号掩体壕): A city-designated historic site since 1995, measuring 21.6 m wide, 14.6 m deep, and 5.4 m high. The front section is poured concrete in formwork; the rear uses soil covered with concrete. A recovered Zero fighter engine and propeller are displayed inside. The surrounding area is maintained as a public park with 7 parking spaces.
• Large-type revetment: One surviving medium/large shelter measuring approximately 9 m high, 43 m wide, and 23 m deep — reputedly the largest surviving aircraft revetment in Japan — originally housing twin-engine Mitsubishi G4M (Type 1 Land Attack Aircraft).
• 9 small revetments: Scattered across rice paddies, these sheltered Zero fighters, Aichi D3A dive bombers, and Nakajima B5N torpedo bombers.
• Parachute maintenance building: A brick structure retaining machine-gun strafing scars on its walls.
• Radio receiver station: A reinforced concrete blast-proof building with 45 cm-thick walls.
• Engine test stand: Concrete equipment pads with overhead suspension rails for running up aircraft engines.
• Bakudan-ike (Bomb Crater Lake): A water-filled crater from the April 21 raid, with an observation deck overlooking the former runway area.
• Flower Road 2: The former main runway, now a tree-lined avenue planted with American dogwood (hanamizuki) and lined with kamikaze pilot memorial monuments.

The Usa City Peace Memorial Museum (宇佐市平和資料館), opened in 2013, houses a full-scale Zero fighter replica and extensive exhibits on the air base's history. The city is planning a larger Usa City Peace Museum (宇佐市平和ミュージアム) to further consolidate preservation and education efforts. Virtual-reality tours of the Takai Underground Shelter (高居地下壕, a 146-meter tunnel with 5 branches) are also available.