An Ingenious System Using Natural Cold Air
Cooling Silkworm Eggs without Mechanical Power
The most remarkable feature of Arafune Wind Cave is that it used naturally occurring cold air rather than artificial refrigeration or mechanical power to preserve silkworm eggs.
A wind cave is a natural phenomenon in which cold air emerges from spaces between rocks, particularly on mountain slopes covered with layers of stones. During winter, cold air and ice accumulate within the rock layers. Even after temperatures rise in summer, the stored cold is gradually released, allowing the area to remain surprisingly cool.
The people who developed Arafune Wind Cave took advantage of this natural phenomenon by constructing carefully arranged stone walls with spaces that allowed the cold air to circulate efficiently.
A Three-Level Storage Structure
The storage facility was designed to prevent the cold air from escaping. Its interior consisted of a three-level structure, with two underground levels and one level above ground.
Silkworm eggs were stored according to their intended breeding season, including eggs for spring silkworms and summer-autumn silkworms. When the eggs were ready to be shipped, they were gradually exposed to outside temperatures so that they could adjust before being transported.
This sophisticated system enabled the facility to control the timing of silkworm hatching using nothing more than the natural cooling power of the mountain.
How Wind Caves Transformed Sericulture
From One Silkworm Crop to Several
Until the Edo period, silkworm farming was generally carried out once a year, in spring. However, as Japan’s silk exports expanded during the Meiji period, increasing silk production became an important economic goal.
Wind caves provided an innovative solution. By keeping silkworm eggs at low temperatures, farmers could delay hatching and control when the silkworms emerged.
This made it possible to raise silkworms two or even three times a year. Summer-autumn silkworm production was particularly valuable because it could provide farmers with additional income during periods when agricultural work was less demanding.
Cooling the eggs for a controlled period also helped synchronize hatching, making the work of raising silkworms considerably more efficient.
One of Japan’s Largest Silkworm-Egg Storage Facilities
A survey conducted in 1909 reported that Arafune Wind Cave had a storage capacity of approximately 1.1 million trays of silkworm eggs.
Its business extended far beyond the local area. At its peak, the facility handled silkworm eggs from 32 prefectures and two urban prefectures, as well as from the Korean Peninsula.
This made Arafune Wind Cave a major hub within Japan’s rapidly developing modern sericulture industry.
Shunshukan and the Development of the Local Silk Industry
The Arafune Wind Cave facility was operated by Shunshukan, an organization established by Shizutaro Niwaya.
The name expressed a desire to improve the quality of silkworms raised in autumn, which had traditionally been considered inferior to spring silkworms, and bring them closer to the quality of spring varieties.
Shunshukan not only stored silkworm eggs but also produced them and handled sales on behalf of others. A branch school of Takayama-sha, an important center of Japanese sericultural education, was also established in connection with the operation.
Through these activities, the facility contributed not only to egg storage but also to the improvement of sericulture techniques in the surrounding region.
An Advanced Transportation and Communication Network
Arafune Wind Cave was located deep in the mountains, while the organization’s offices were located closer to the settlements. To connect the two, a dedicated telephone line was installed, an advanced communication system for its time.
Transportation also made use of railways, horse-drawn vehicles, and automobiles.
These arrangements demonstrate the scale and sophistication of the operation. Arafune Wind Cave was not simply an isolated mountain storage facility but part of a much larger commercial and industrial network.
Decline, Preservation, and World Heritage Recognition
The importance of wind caves gradually declined as new technologies were introduced. Artificial methods of inducing silkworm hatching and refrigeration using ice eventually provided more controllable alternatives.
Silkworm-egg storage at Arafune ended during the Showa period, and by the 1950s the buildings that once stood over the stone structures had disappeared.
What remains today are primarily the carefully constructed stone foundations and storage structures.
From Industrial Heritage to World Heritage
Despite the disappearance of the original buildings, the site was recognized as an important surviving example of Japan’s modern silk industry.
In 2010, the Arafune and Higashitani Wind Cave Silkworm Egg Storage Sites were designated a National Historic Site.
In 2014, Arafune Wind Cave became one of the component sites of the UNESCO World Heritage property associated with the Tomioka silk industry heritage. Its international recognition reflects the importance of the technological and industrial systems that supported Japan’s silk production during the country’s modernization.
Higashitani Wind Cave
Higashitani Wind Cave, located in Nakanojo Town, Gunma Prefecture, is another historic wind-cave site included in the National Historic Site designation.
It was developed in 1906 by Senogoro Okugi and later became the second-largest facility of its kind in Gunma Prefecture.
While Arafune Wind Cave served a wide network extending across Japan and overseas, Higashitani Wind Cave primarily supported sericulture in the Agatsuma region. It therefore represents another way in which wind-cave technology contributed to local communities.
Higashitani Wind Cave has not been developed extensively as a tourist destination, but it remains an important site for understanding the history of modern Japanese sericulture.
Experiencing Arafune Wind Cave Today
Wind Cave Cold-Air Experience Center
In 2018, the Wind Cave Cold-Air Experience Center opened near the site, allowing visitors to experience the naturally cool air emerging from between the rocks.
Even on a hot summer day, the air around the wind cave can feel remarkably cool. Experiencing this natural phenomenon helps visitors understand both the power of the environment and the ingenuity of the people who learned how to make practical use of it.
Access and Visiting Information
The nearest railway station is Shimonita Station on the Joshin Electric Railway. From the station, the site can be reached by car in approximately 30 minutes.
Because Arafune Wind Cave is located in a mountainous area, visitors should wear comfortable walking shoes and bring clothing suitable for cooler conditions.
Mobile phone reception can be limited in some areas, and visitors should take care not to disturb the surrounding natural environment.
The site is generally closed during winter, from December through March. Visitors should check the latest opening information before planning their trip.
Following the Heritage of Japan’s Silk Industry
A visit to Arafune Wind Cave becomes even more meaningful when combined with other sites connected with the development of Japan’s modern silk industry.
These include the Tomioka Silk Mill, the Tajima Yahei Sericulture Farm, and the Takayama-sha Sericulture School. Together, these sites illustrate different stages of the silk-production system, from the development of high-quality silkworm eggs and improvements in sericulture techniques to the preservation and distribution of silkworm eggs.
Standing quietly in the mountains, the stone structures of Arafune Wind Cave may appear simple at first glance. Yet they once formed part of an extensive industrial network connecting rural Gunma with communities throughout Japan and even overseas.
A Natural Refrigerator Preserving Japan’s Industrial History
Arafune Wind Cave was a remarkable natural refrigerator that helped transform Japanese sericulture during the country’s modernization.
Its ingenious use of naturally occurring cold air, enormous storage capacity, nationwide business network, and eventual recognition as World Heritage all demonstrate the creativity and determination of the people who developed the facility.
Today, the cold air still flows quietly through the rocks, just as it did more than a century ago. A visit to Arafune Wind Cave offers travelers a unique opportunity to experience the meeting point of nature, traditional knowledge, technology, and Japan’s modern industrial history.