Otsu Fu-Go Balloon Bomb Launch Base Site

The Otsu Balloon Bomb Launch Base (大津放球基地) in Kitaibaraki, Ibaraki Prefecture, was the headquarters and primary launch site for Operation Fu-Go (ふ号作戦) — Imperial Japan's audacious attempt to strike the North American mainland using unmanned hydrogen balloons carrying incendiary and antipersonnel bombs.

The Fu-Go (風船爆弾, fūsen bakudan, literally "balloon bomb") was a weapon conceived by the Imperial Japanese Army's Ninth Military Technology Research Institute (Noborito Research Institute) in collaboration with meteorologists who had discovered the jet stream over the Pacific. Each balloon was approximately 10 meters in diameter, constructed from multiple layers of washi (traditional Japanese paper made from mulberry bark) laminated with konnyaku (konjac) paste for gas-tightness. Filled with hydrogen, it carried a payload of one 15 kg antipersonnel bomb and two 5 kg incendiary devices, along with an ingenious altitude-control mechanism using sandbag ballast and barometric-triggered gas release valves.

Three coastal launch bases were established along the Pacific shore of eastern Honshu to exploit the winter jet stream flowing eastward toward North America: the First Battalion at Otsu (Ibaraki Prefecture, present-day Kitaibaraki), the Second Battalion at Ichinomiya (Chiba Prefecture), and the Third Battalion at Nakoso (Fukushima Prefecture, present-day Iwaki). The Otsu base was the largest, serving as regimental headquarters (the regiment as a whole numbered about 2,000 men), with the First Battalion's three companies operating 18 launch pads (27 by some accounts). Uniquely, Otsu possessed its own hydrogen gas generation plant, while the other bases relied on gas transported from factories around Tokyo.

The campaign opened on November 3, 1944, and continued until April 1945, when launches from Otsu were finally halted. Approximately 9,300 balloons were released in total from the three sites, timed during the winter months when the jet stream was strongest. The balloons took roughly 30 to 60 hours to cross the Pacific at altitudes of 9,000 to 11,500 meters. An estimated 300 to 1,000 reached North America, with confirmed landings or sightings in 26 U.S. states, Alaska, Canada, and Mexico. On May 5, 1945, a balloon bomb killed six civilians — a pastor's wife and five Sunday school children — near Bly, Oregon, the only enemy-caused casualties on the U.S. mainland during the war.

The Otsu base was the only one of the three that operated continuously until the campaign's conclusion. Today, one of the original 18 concrete launch pads (放球台) survives — a circular platform approximately 16 meters in diameter, partially overgrown but still visible among the trees near the coastal road between the Ibaraki Prefectural Tenshin Memorial Museum of Art (五浦美術館) and Hirakata Port. Two memorial stones stand nearby: the "Wasureji Peace Monument" (わすれじ平和の碑), inscribed with an account of the balloon bomb operations, and a separate "Soul Repose Monument" along the roadside. The site has no formal visitor facilities or signage, but is freely accessible on foot.