Hiyoshidai Underground Bunker (Keio University)

The Hiyoshidai Underground Bunker is a network of interconnected tunnels excavated beneath the hills of Keio University's Hiyoshi Campus in Kohoku Ward, Yokohama, Kanagawa Prefecture. In 1944, as the war situation deteriorated and the threat of Allied air raids on Tokyo grew, the Imperial Japanese Navy began moving command facilities out of central Tokyo, at first into borrowed Keio school buildings and dormitories (Naval General Staff Section 3 moved in from March 1944). The elevated terrain of the Hiyoshi Campus—35 to 40 meters above sea level—and its proximity to both Yokosuka Naval Base and central Tokyo made it an ideal choice.

Construction of the underground complex began in the summer of 1944, carried out by naval construction battalions and Korean forced laborers. Working under extreme urgency, the tunnels were carved through the hillside with hand tools and basic equipment. The Combined Fleet Headquarters under Admiral Soemu Toyoda moved into the Keio dormitory above the tunnels on September 29, 1944, and the core underground rooms — operations, telegraph and cipher — came into use from about November. The Navy's most critical command organs were consolidated underground: the Combined Fleet Headquarters occupied the main section (known as the Mamushi Valley or まむし谷 entrance), while the Naval General Staff Section 3, the Aeronautical Headquarters, and the Personnel Bureau each had their own tunnel networks.

The complex is usually described as extending approximately 2.6 kilometers (Keio University puts the total at more than 5 kilometers when all the bunkers and smaller tunnels are counted), reaching depths of about 30 meters below the surface. The tunnels featured reinforced concrete walls, dedicated rooms for the commander-in-chief, operations planning, communications, and cipher work. From this subterranean headquarters, Japan's naval leadership directed some of the war's most consequential and devastating engagements: the Battle of Leyte Gulf (October 1944), the defense of Iwo Jima (February–March 1945), and the Battle of Okinawa (April–June 1945). The orders dispatching kamikaze special attack units were also transmitted from the communications facilities within these tunnels.

Following Japan's surrender on August 15, 1945, the tunnels were sealed and the campus was returned to Keio University. For decades, the tunnels remained largely forgotten and inaccessible. In the 1980s, local residents and former students who had discovered tunnel entrances formed the Association for the Preservation of the Hiyoshidai Tunnels (日吉台地下壕保存の会). The association has since worked to document, preserve, and publicize the tunnels, conducting regular guided tours open to the public.

A government survey of approximately 850 World War II-related sites ranked the Hiyoshidai Tunnels as Grade "A"—described as "a historic site essential to the understanding of our modern national history." Despite this recognition, preservation has been an ongoing challenge, as sections of the tunnels have been damaged or destroyed during campus development projects. The site remains one of the most significant and tangible reminders of how Japan's wartime military infrastructure was embedded within civilian institutions.