Narita's Secret Imperial Shelter Survives More Than 80 Years Beneath Sanrizuka Park

A Wartime Refuge Hidden Near Narita's Runways

A shelter completed on December 8, 1941, the day the Pacific War began, still lies 5.2 meters beneath Sanrizuka Memorial Park in Narita, Chiba Prefecture. Built in secrecy for then Crown Prince Akihito, the imperial air raid shelter known as the Obunko has survived more than eight decades without a single crack, condensation stain or sign of structural fatigue.

The Japanese government designated the shelter a registered tangible cultural property in 2024, recognizing its historical and engineering significance. Its concrete walls, about 70 centimeters thick, and an entrance door weighing 5 tons reflect the extraordinary effort made to protect the imperial family from wartime bombing.

Akihito never used the shelter. He was evacuated elsewhere during the war, leaving this carefully prepared refuge to take on other purposes after Japan's defeat. It became a storage space for agricultural produce before opening to the public in 2011.

Today, visitors can descend from the park's quiet lawns into a structure where even the roar of aircraft using nearby Narita International Airport cannot be heard. History enthusiasts, construction professionals and curious travelers come to examine it, yet the shelter remains little known even among people living in Narita.

Why the Imperial Stock Farm Was Chosen

The Obunko was built when Japan was preparing for the dangers of modern warfare. Air raids on cities had become a frightening possibility, and protecting the imperial family was a matter of national concern. The shelter was intended to provide a secure place for the crown prince if refuge became necessary.

The site was then the Shimofusa Imperial Stock Farm. Remote, secure and already under imperial control, it offered a suitable location for a construction project that had to remain secret. The grounds now form part of Sanrizuka Memorial Park, close to the airport's busy runways.

Its completion on the first day of the Pacific War gives the building a particularly striking place in the wartime chronology. The refuge was ready as the conflict began, but the crown prince's evacuation elsewhere meant that its intended occupant never took shelter there.

What Visitors Find Underground

The shelter covers about 69 square meters and consists of a main room flanked by two antechambers. An antechamber is a smaller room before or beside the principal room. Here, thick iron doors separate the chambers, forming barriers designed to withstand the shock waves from nearby explosions.

The main room occupies about 11 square meters. Although small, it was carefully insulated from the outside world. The silence inside is one of its most striking features: aircraft noise from Narita International Airport does not reach the room.

Katsuhiro Itokawa, a guide who leads tours of the site, notes that the temperature inside remains steady at 19 degrees Celsius, even when summer temperatures outside reach 28 degrees. Those figures put the interior 9 degrees below the outdoor temperature in that example.

Lights, electrical outlets and telephone jacks remain part of the shelter's equipment. Together, they show that planning extended beyond protection from explosions. The space was also prepared for lighting, power and communication, with the comfort and safety of the imperial family in mind.

Company Records Reveal the Construction Details

For decades, the methods used to build the Obunko remained obscure. Records from Hazama Corp., now Hazama Ando Corp., eventually brought the construction story to light. The company was responsible for the project, and the records appeared in its centennial history published in 1989.

The account describes a dense framework of steel reinforcing rods inside the concrete. Reinforced concrete combines concrete with steel to make a stronger structure. At Narita, rods measuring 25 millimeters thick were bound together at intervals of 10 centimeters, creating a tightly arranged steel skeleton.

The construction details recorded in that history include:

• An entrance door weighing 5 tons, installed to withstand blasts and prevent unauthorized entry.
• Steel reinforcing rods 25 millimeters thick, bound together at intervals of 10 centimeters.
• Reinforced concrete walls about 70 centimeters thick, comparable in thickness to those used in modern skyscrapers with 30 stories.
• No exposed nails, minimizing weak points and supporting durability.

The heavy entrance door and the thick internal iron doors served related purposes. The entrance guarded access and resisted blasts, while the doors between rooms provided further separation from the shock waves of nearby explosions.

Eight Decades Without Cracks or Leaks

The Obunko remains free of cracks, leaks and condensation after more than 80 years underground. Its condition is a central part of the site's engineering interest, alongside the detailed construction methods preserved in the contractor's records.

Condensation occurs when moisture in the air turns into water on a cooler surface. Underground structures can be places where dampness is readily noticed, making the absence of condensation stains an important part of the shelter's reported condition.

Construction professionals travel from across Japan to study the building. The combination of thick concrete, densely arranged reinforcement and carefully finished construction gives them a surviving example of work intended to resist severe external forces and remain durable.

The engineering also shapes the visitor's experience. Heavy doors, a small main room, a cool interior and near silence make the shelter feel both practical and fortresslike. These features bring the protective purpose of the building into focus without requiring visitors to understand technical drawings.

The Imperial History Above the Shelter

The Shimofusa Imperial Stock Farm had an imperial role long before the shelter was built. Before the construction of Narita International Airport, the area was a sprawling estate devoted to breeding horses for the imperial family.

The nearby Museum of Imperial Horse Husbandry preserves that earlier history. Managed by Narita City and staffed by knowledgeable volunteer guides, it displays artifacts including royal carriages and airplane equipment used by Emperor Showa.

Guided visits often include the air raid shelter, connecting the museum's account of the estate with the wartime preparations beneath it. Visitors can learn how the same location supported the imperial household through horse breeding and later became a place chosen for the crown prince's protection.

The park, museum and shelter therefore preserve different parts of the site's history. The shelter's position beneath the former farm helps explain why it was built there, while the museum gives visitors a broader view of the imperial activities once carried out on the grounds.

From Secret Refuge to Public Access

After Japan's defeat, the Obunko was used to store agricultural produce. For decades, it remained largely forgotten, rarely discussed and known chiefly as a storage space rather than a historic site open to visitors.

The opening in 2011 changed that relationship. People could enter the shelter and consider the wartime anxieties behind its construction. Its main dates trace a journey from secrecy to public recognition:

• December 8, 1941: The shelter was completed on the day the Pacific War began.
• After Japan's defeat: The unused imperial refuge became a storage space for agricultural produce.
• 1989: Hazama Corp.'s centennial history published records describing its construction.
• 2011: The shelter opened to the public.
• 2024: The Japanese government designated it a registered tangible cultural property.

The shelter now draws a steady stream of history enthusiasts, engineers and travelers from Japan and abroad. Local awareness remains limited, however, reflecting the long period during which its existence was secret or received little public attention.

Efforts to raise awareness and preserve the structure continue. The 2024 designation marked a major step in recognizing the shelter as a place where future generations can learn about wartime history and construction.

Remembering People Beyond the Imperial Household

The Obunko records the extraordinary measures taken to protect the imperial family. Its preservation also creates a place to consider Japan's wider wartime experience, including the fear of air raids, the displacement of families and the suffering of children who lost their parents.

As fewer people remain who remember the anxieties and sacrifices of that period, shelters, museums and memorials help carry those memories forward. Historians regard the preservation of such sites as essential to understanding the full impact of war on Japanese society.

An academic study discussing war orphans, known in Japanese as sensō koji, connects their experiences to wartime events, failures of state protection and the silence that continued around their suffering:

War orphans are products of war, not just postwar Japan, and their experiences highlight the failures of state protection and the enduring silence surrounding their suffering.

Narita City's work to maintain the shelter and admit visitors helps preserve a physical connection to that era. The building gives people a concrete place from which to consider both the preparations made for imperial safety and the wider human experiences of war.

Other Imperial Shelters and Narita's Role Today

Narita's Obunko is not the only surviving imperial air raid shelter from World War II. The Tamozawa Imperial Villa in Nikko also has a garden shelter built to protect members of the imperial family during air raids. Visitors can still see it today.

The villa, once a summer residence for Emperor Taisho, has been restored and opened as a cultural site. Like the shelter at Narita, its wartime refuge provides a view of the fears and priorities that shaped preparations for the imperial household.

Many other shelters have disappeared or remain inaccessible. Narita's preserved structure offers a rare opportunity to enter a surviving refuge and examine the rooms, doors and equipment intended to keep its occupants safe.

For guide Katsuhiro Itokawa, the Obunko serves as a living classroom, with lessons in history, engineering and human resilience. Its modern role brings together a building that has endured underground, a once secret imperial project and a public effort to keep wartime memory accessible.

Key Points

• The Obunko in Narita was completed for then Crown Prince Akihito on December 8, 1941, but he never used it.
• The shelter lies 5.2 meters underground and covers about 69 square meters, including a main room of about 11 square meters.
• Its construction includes walls about 70 centimeters thick, densely arranged steel reinforcement and an entrance door weighing 5 tons.
• After more than eight decades, it remains free of cracks, leaks, condensation stains and signs of structural fatigue.
• Used for agricultural storage after the war, it opened to the public in 2011 and received registered tangible cultural property status in 2024.
• The shelter and nearby museum preserve the wartime and imperial history of the former Shimofusa Imperial Stock Farm.