In the waning months of the Second World War, as it was becoming increasingly clear that the Americans were going to win the Pacific War, the Japanese Army resolved to build a vast underground shelter for the nation's leaders, and especially the Imperial family, in case the bombing would intensify on Tokyo. The bunker was designed to withstand B-29 bombings and would serve as an alternative headquarters for the Imperial General headquarters and all ministries. More surprisingly it was to include a full-scale subterranean palace, out of consideration for the emperor's comfort. The location chosen was the mountain flank of Matsushiro, 8 km due south of Nagano.
The complex is astounding by its sheer scale. Tunnels run for 10 km (7 miles), cover nearly 6,000 square kilometres of floor-space. Yet, only 75% of the base was complete when Japan surrendered on 15th August 1945. The human cost was terrifying too. Local Japanese residents were joined by 7,000 to 10,000 Korean slave laborers and toiled for months in inhumane conditions to dig the tunnels through hard rock. An estimated 300 to 1,500 of them died of malnutrition, exhaustion and executions, mostly Koreans.
Construction commenced in November 1944, but Emperor Hirohito was not informed until May 1945. The emperor presistently refused to relocate as he feared that the Army was trying to isolate him in order to have full control on the military operations. Hirohito feared above all that the Army would seek to pursue the war at all cost and to suicidal extremes.
Nagano City opened small section of the caves to the public in 1985, following pressure from the local community. Visitors are able to glance through the first 500 metres of tunnels and shafts in Mount Zōzan. The entrance is located less than 1 km south of Matsushiro station. If you do come by train, the remains of Matsushiro Castle are just next to the station (north-west).
Map of attractions in Nagano