Former Shimousa Imperial Ranch (Sarizuka Memorial Park) Air

Finished on the Day the Pacific War Began

The Air-Raid Shelter of the Former Shimofusa Imperial Stock Farm (旧下総御料牧場（三里塚記念公園）防空壕, Kyū Shimofusa Goryō Bokujō Bōkūgō, also transliterated Kyu Shimousa Goryo Bokujo Bokugo) is a reinforced-concrete underground structure of 1941 in Sanrizuka Memorial Park, Narita City, Chiba Prefecture, entered on Japan's Register of Tangible Cultural Properties on 3 December 2024.

It was completed on 8 December 1941, the day Japanese forces attacked Pearl Harbor.

The commission ran through the machinery of the imperial household rather than the military. Japan's Air Defence Law of 1937 had set off a wave of shelter construction across the country, and in September 1941 the Takumiryō, the Imperial Household Ministry's building office, placed an order with the contractor Hazama, now Ando Hazama. The firm's own centenary history covering 1889 to 1945 lists the job under a title naming it as shelter work at Sanrizuka for the crown prince. That prince was Akihito, then seven years old, later emperor and now emperor emeritus. The Agency for Cultural Affairs record phrases the attribution carefully, as something handed down rather than documented in full, and this article follows that caution.

Three months from order to completion is fast for a structure of this weight. The schedule reads as what it was: a job carried out under pressure, in the weeks before a war that had already been decided on.

An H on the Ground, Under a Mound

From the surface the shelter announces itself as a low grassy mound behind the Kihinkan guesthouse, with concrete stairs cut into it. The structure sits beneath that earth cover, and the earth is part of the design rather than later landscaping; the official record describes the building as a reinforced-concrete basement with fill above.

The plan is an H. A main chamber occupies the crossbar, with anterooms in front of it and passages running east and west to form the uprights. Two flights of stairs descend from ground level into the anterooms, and two blast vents relieve pressure. Anyone who has walked through wartime shelters elsewhere will recognise the logic: no straight line runs from an entrance to the room where people sit, so a blast wave loses its force turning corners before it arrives.

The numbers are what hold the attention. Walls to the main chamber are 700 millimetres thick. The steel doors sealing the boundary between the anterooms and the passages are about 120 millimetres of plate. The whole works covers 69 square metres, of which the main chamber accounts for a little over eleven, roughly the floor area of a small bedroom, wrapped in something closer to a bank vault.

Concrete of that period is usually the weak point in surviving wartime structures, since aggregate and cement quality fell away sharply as the war went on. Here the opposite proved true. When the interior was surveyed in 2008, the concrete tested at strengths in the range of 45 to 69 newtons per square millimetre, comparable to what goes into high-rise towers of thirty storeys and above today. The shelter was built at the very start of the war, before shortages bit, and by a major contractor working for the imperial household. It shows.

Sealed, Forgotten, and Opened Again

The shelter is understood never to have been used for its purpose. No air raid drove anyone down those stairs, and after 1945 the structure simply sat in the grounds of a working stock farm with no job to do.

Around 1975 the entrances were closed off for safety, weighted with concrete slabs and covered with soil. For more than three decades the mound was a bump in the lawn that visitors walked past. The investigation of 2008 came about through the persistence of local history enthusiasts, and it is that survey which produced the concrete-strength figures. Conversion work for public viewing followed in 2010, and the interior has been accessible since.

Descend and the temperature drops. There is no climate control, and the mass of concrete and earth holds the interior at a fairly constant cool through the year, so a light layer is worth carrying even in August. The main chamber is bare. Lighting is discreet, the surfaces are unpainted, and the steel doors still swing on their hinges. Standing in a room built to survive an attack that never came, for a child who never came either, is a strange experience and a quiet one.

Viewing is arranged through the Sanrizuka Imperial Stock Farm Memorial Hall in the same park, and a member of staff accompanies visitors and explains the structure. There is no charge. The memorial hall opens from 9:00 to 16:30 and closes on Mondays, on the following Tuesday when a Monday is a public holiday, and from 29 December to 3 January; calling 0476-35-0442 ahead of a special trip is wise, since access depends on staff being free to escort you. The park itself is free and open, and the Kihinkan guesthouse, registered on the same day, stands a few paces in front of the mound.

Why It Was Registered

Japan protects historic buildings under two separate systems. Designation applies rigorous controls to a limited number of properties of the first rank. Registration, established in 1996, is a lighter framework covering a much wider body of buildings from the Meiji period onward. The shelter was registered under criterion (3), covering works that would not be easy to reproduce, with registration number 12-0327 in the 108th round.

That criterion fits the object well. The scarcity here is not aesthetic. Civilian and imperial air-raid shelters of this quality were mostly demolished, buried, or left to rot, and one that survives intact with its steel doors, its blast vents, and a documented paper trail back to the contractor is unusual. Ownership rests with the Ministry of Finance, a legacy of the land passing into national hands, while Narita City manages the park around it.