During the war, underground air raid shelters were built in at least 9,850 locations across Japan. Currently as many as 487 of these locations are at risk of collapse, causing damage to houses above or serious injuries or death to local residents.
On November 29th, the collapse of a former Japanese army shelter in Tachikawa, T0kyo, caused serious property damage to the landowners house. The Tokyo District Court ordered state reparations to compensate the owner.
The Ministry of Land, Infrastructure and Transport (MLIT) conducted a factual investigation in 2009 of underground shelters. The area with the largest concentration of air raid shelters was Kagoshima with 2,813, of which 49 were considered high risk. In the Kanto Region, Kanagawa had the highest number with 510 shelters of which 9 were high risk. The MLIT lists 169 locations in Tokyo of which 9 pose a hazard. There is one in Minato-ku (possibly under Motoazabu) which is considered safe.
The underground shelters were constructed during the war by the Japanese army for munition factories, but were primarily used as protection from air raids.
In Kagoshima in 2005, four junior high school students died from carbon monoxide poisoning at the site of a former air-raid shelter. After the incident the closed up the entrances to over 1,000 shelters with concrete blocks, but 460 still remain unsealed.
In 2000, again in Kagoshima, a woman died when a prefectural road suddenly caved in. The bereaved family sought compensation from the country and prefecture and were awarded 56 million Yen.
The MLIT and a self-governing body conducted a study from 1998~2009 to assess the risk that these underground shelters have on surrounding buildings and spent 5.3 billion Yen filling in 195 shelters. However, more shelters continue to be discovered but are not filled in due to the high costs required.
According to a source at the MLIT, as these shelters are discovered there is a chance that the appraised value of the land will decrease. There may also be landholders who know about a shelter but do not want to make it public for this reason, so it becomes very difficult to ascertain the exact number of shelters.
Source: The Asahi Shimbun, November 28, 2010