Tateyama Naval Air Group Ruins (Akayama Underground Tunnels)

The Tateyama Naval Air Group (館山海軍航空隊) was established on June 1, 1930, as the fifth operational naval air unit in Japan, following Yokosuka, Sasebo, Kasumigaura, and Omura. Situated at the entrance to Tokyo Bay on the southern tip of the Boso Peninsula in Chiba Prefecture, the base was built on land reclaimed from shallows uplifted by the 1923 Great Kanto Earthquake. Under the Yokosuka Naval District, it initially operated carrier-based attack aircraft and seaplane reconnaissance aircraft.

During the 1930s the base served as a key element of Tokyo Bay's defense network. When the Kisarazu Naval Air Group was established in 1936 for long-range offensive operations, Tateyama transitioned to a homeland defense role. Throughout the Pacific War, the base hosted various aircraft including Mitsubishi G3M medium bombers, Mitsubishi A5M fighters, Nakajima B5N torpedo bombers, and later the Kyushu Q1W anti-submarine patrol aircraft.

The Akayama Underground Tunnels (赤山地下壕) represent the most significant surviving structure from the wartime base. Few records survive and the date of construction is uncertain, but testimony from those involved indicates that work began in the final phase of the Pacific War, driven by the urgent need for bomb-proof facilities as American air raids intensified. The tunnels extend approximately 1.6 kilometers through the volcanic tuff of Mount Akayama, with an unusually dense layout — passages are spaced only 5-10 meters apart, compared to the standard 10-20 meters for military shelters. The complex housed a self-generating power station, aircraft repair workshop, emergency hospital, communications room, and command offices. Construction continued until Japan's surrender on August 15, 1945, and the tunnels were never fully completed.

The geological strata visible inside the tunnels — including overlapping layers and fault displacement phenomena — also make the site valuable as natural heritage. The tunnel walls bear the marks of both hand-digging and mechanical boring, offering insight into the desperate pace of wartime construction.

After Japan's surrender, about 3,500 troops of the U.S. Army's 112th Cavalry Regimental Combat Team landed at Tateyama on September 3, 1945 and occupied the area. It was returned to the Japanese government in 1953 and assigned to the Japan Maritime Self-Defense Force (JMSDF), becoming the birthplace of postwar Japanese naval aviation. JMSDF Fleet Air Wing 21 has been headquartered at Tateyama since 1961, and the base now operates as a major helicopter base.

The Akayama tunnels were opened to the public on April 1, 2004. Of the total 1.6 km length, approximately 250 meters are accessible to visitors (the tunnels have been temporarily closed since a partial collapse on the visitor route was found in November 2025). The site is designated as a Tateyama City Historic Site and stands as one of the largest publicly accessible wartime underground complexes in Japan.