Yoshimi Hyakuana (Underground Munitions Factory Ruins)

Yoshimi Hyakuana (吉見百穴, literally "Yoshimi Hundred Caves") is located on a tuff hillside in Yoshimi Town, Hiki District, Saitama Prefecture, approximately 50 kilometers northwest of central Tokyo. The site preserves 219 yokoana (lateral cave tombs) carved into the soft volcanic rock during the late Kofun period (6th–7th century AD), making it one of the largest concentrations of such tombs in Japan; Tsuboi's excavation uncovered 237 caves, more than ten of which were later destroyed by the wartime tunnelling. First scientifically investigated in 1887 by Shogoro Tsuboi of Tokyo Imperial University—who initially believed them to be prehistoric dwellings—the caves were correctly identified as burial chambers by the 1920s and designated a National Historic Site in 1923. The site also harbors hikarigoke (luminous moss, Schistostega pennata), designated a National Natural Monument in 1928.

The site's wartime chapter began in late 1944 when the Imperial Japanese military, desperate to protect aircraft production from devastating American strategic bombing, selected the Yoshimi hills for an underground factory complex. The Nakajima Aircraft Company's engine works at Musashino in western Tokyo had been bombed repeatedly from November 1944, and it was decided to move the company's still-undamaged Ōmiya plant underground. The military ordered the construction of a subterranean facility at Yoshimi to manufacture aircraft engines—specifically components for the Nakajima Ha-45 (Homare) radial engine that powered fighters such as the Ki-84 Hayate and the N1K2-J Shiden-Kai.

Construction — carried out in 1944–45 under the designation "Yoshimatsu Works" (吉松工事) — covered some 1,300 meters of hillside between the Hyakuana and the site of Matsuyama Castle. The tunnels were designed in a grid (go-ban-no-me) pattern with lateral and longitudinal passages intersecting at regular intervals, each approximately 3–4 meters wide and 2–4 meters high. The planned total tunnel length was approximately 8,400 meters, with a factory floor area of roughly 10,000 tsubo (33,000 square meters). An estimated 3,000 to 3,500 Korean laborers were mobilized from across Japan—many previously conscripted for coal mine work—and forced to work around the clock in grueling conditions. The tunneling destroyed more than 10 of the nationally designated cave tombs.

By the war's end, approximately 80% of the tunnel excavation had been completed. Machinery was brought in around July 1945 and limited engine parts manufacturing began, but the factory never achieved full-scale production before Japan's surrender on August 15, 1945. The facility represents one of numerous desperate "underground dispersal" (地下疎開) projects undertaken across Japan in the final year of the war, alongside similar underground factories at Hino (Tama), Utsunomiya, and elsewhere.

After the war, the tunnels were sealed. Today the site is managed by Yoshimi Town and open to visitors (8:45–16:50, last entry 16:30; adult admission ¥300). The ancient cave tombs are openly accessible on the hillside, and several of the wartime tunnel entrances—massive circular openings approximately 3 meters in diameter—are visible but closed off with iron gratings because of the risk of collapse. The contrast between the small, carefully carved 6th-century burial niches and the brutal industrial tunnels blasted through the same hillside offers a visceral lesson in how wartime desperation can reshape even the most ancient of landscapes.