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Joshin’etsukogen NP, Gunma
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The rusty deposits are not only relics of mining: iron bacteria and Chatsubomi-goke still help precipitate iron in the acidic stream. Their biological mineral-making helped form the ore that miners once extracted.
Carbon in the cliffside iron deposits has been dated to more than 10,000 years ago. Fossil moss and bacterial traces preserved in the ore show this unusual iron-forming ecosystem was already active in deep prehistory.
Anajigoku means “Hell Pit” and once referred to a huge, water-filled hollow. Iron mining removed that original hollow, yet its name survived for the mossy site visitors see today.
At its height, the mine settlement had more than 2,000 residents, including about 300 children. It had a theater, shops, film screenings and sports days—a whole community in these remote mountains.
By June, Japanese black salamander larvae swim in a nearby pond, while forest green tree frogs leave fluffy white egg masses on branches above the water—a cottony nursery suspended over the pool.
The JR Agatsuma Line has mining origins: ore traveled by twin aerial ropeways 7.5km to Oshi Station, with capacity for 600 tonnes a day. After residents pressed for service, the Taishi branch began carrying passengers in 1954.
Despite its name, Chatsubomi-goke is a liverwort, not a true moss. It thrives in water with pH 2.5–2.9, and the Japan Bryological Society identifies Anajigoku as home to Japan’s largest colony.
Measurements near Anajigoku recorded spring-water temperatures of 19.6–29.5°C, which scarcely fall even when winter air drops below freezing. The warm, moss-lined flows stand out against the snow.
During Japan’s postwar rebuilding, Gunma Iron Mine ranked second nationally after Kamaishi. In July 1949 it produced 20,000 tonnes in a month, and lifetime shipments reached about 3 million tonnes before it closed in 1965.
Chatsubomi is part of the 887-hectare Yoshigadaira Ramsar Wetlands, a volcanic mountain sequence that stretches from the 2,160m Kusatsu-Shirane crater down to the park at about 1,200m.
Japan’s 2017 Natural Monument designation protects a 440m corridor from Anajigoku along the Motoyama River to below Onsen Otaki—including the moss, iron-forming waters, iron deposits and nearby amphibian habitat.
After mining ended, the former owner kept the land as a staff recreation ground. It was transferred to Nakanojo Town in April 2012, turning this old industrial property into the public park visitors know today.
Among the mining traces is the site of a shrine built during the Gunma Iron Mine years so workers could pray for safety—a quiet reminder that this striking natural landscape was also a workplace.
