Chatsubomigoke Park

Getting the most out of Chatsubomigoke Park is less about the destination than about timing the light right.

At the far western edge of Nakanojo, in a basin formed by eruptions of Kusatsu-Shirane, warm acidic springs seep out of the ground and a moss called chatsubomigoke grows on them in a continuous glowing green mat. Almost nothing tolerates water this acidic, which is exactly why the moss has the place to itself. The hollow it fills is called Ana-Jigoku, 'hole hell', a name given long before anyone came to admire it.

The site is also an industrial ruin. This was the Gunma Iron Mine, the country's largest source of limonite, worked until it closed in 1966 — and the iron ore was being produced by the same acidic water and iron-fixing bacteria that keep the moss alive. That combination of a rare biological community and an active ore-forming process got the site designated a national natural monument in 2017. A boardwalk loop takes you around and down into the hollow; the moss is at its most saturated after rain and in the low light of early morning.

Chatsubomigoke takes its name from the shape of its clustered growth, which resembles tightly furled tea buds when viewed up close, and it is one of only a handful of moss species known to tolerate the combination of high acidity and elevated iron content found in this kind of spring water. Researchers studying the site have described how the moss and iron-oxidizing bacteria living alongside it interact in a self-sustaining cycle: the bacteria draw energy from oxidizing dissolved iron in the water, and the resulting mineral deposits build up around the moss in a slow process thought to have contributed to the original iron ore deposits mined at the site.

The Gunma Iron Mine that once operated here supplied ore during a period when Japan was expanding its domestic industrial capacity, and its closure reflected a broader shift as cheaper imported ore made small mountain mines like this one increasingly uneconomical across the country. What remains today — rusted mine structures visible from parts of the boardwalk, tailings piles now overgrown, and the acidic seeps that once made the ore workable — gives the park an unusual double identity as both a rare ecological site and a preserved fragment of Japan's twentieth-century mining industry.