Chatsubomi Moss Park
📍 Gunma, Nakanojo
A neon-green stream of rare acid-loving moss fed by a hot mineral spring called Anajigoku — 'the hell pit' — inside a former iron mine deep in the Gunma mountains.
Chatsubomi Moss Park (チャツボミゴケ公園, Chatsubomigoke Kōen) protects one of the strangest wetlands in Japan: a stream bed carpeted in vivid, electric-green moss that thrives in water acidic enough to sting. The moss grows out of Anajigoku (“the hell pit”), a spring born of Mt. Shirane’s volcanic activity, on the site of a former iron mine deep in the mountains above Nakanojo, Gunma.
Why It’s Interesting
Chatsubomi moss is the most acid-tolerant of the roughly 1,800 known moss species on Earth, and this is essentially the only place on Honshu where it grows in the wild. The spring water runs at a pH of roughly 2.6–2.9 — strong enough that iron bacteria and the moss itself precipitate iron ore out of it, a process that once made this a working mine (Gunma Iron Mine, closed in 1965 when the deposit ran out). In 2017, the government designated the site — the spring, the moss colony, and the iron-forming riverbed together — a national Natural Monument. What’s left today is a short, otherworldly walk along a stream that looks less like Japan and more like a nature documentary about extremophiles.
Best Time to Visit
The park’s reception is open mid-April through November only, closed entirely for winter. Fresh green in the early-summer months makes a striking contrast against the moss, and October–November foliage on the surrounding hillside turns the whole scene into a red-and-green tableau. Renge azaleas also bloom near the entrance in late spring.
Getting There
There’s no train or bus service to the park itself — a car is effectively required, roughly 40 minutes from JR Naganohara-Kusatsuguchi Station or 30 minutes from Kusatsu Onsen, with free parking at the visitor center. From there, a free shuttle bus covers the walk to Anajigoku in about 5 minutes, or visitors can take the marked 30-minute trail on foot.
🐿️ Cinnamon was here
Field report
Ghost popsicle sent me just the name and nothing else, which is the best kind of tip — I looked it up and found out "chatsubomi" means "tea bud," a very polite name for moss that grows out of something called Anajigoku, the hell pit.
I rode the shuttle up from the visitor center instead of the 30-minute trail, mostly because I have short legs, and came out at a stream so bright green it looked painted on. The water comes out of the old iron mine acidic enough to sting — I did not drink it, learned my lesson from the paw incident — and the moss just sits there thriving in it anyway, one of maybe two places on the whole island where it grows like this.
Other visitors keep saying it feels like nowhere else in Japan, and I agree — it doesn't look real. My only tip, same as the humans': go in October if you can, when the red-gold hillside makes the green stream look even more impossible.
— Cinnamon 🐿️
📸 Cinnamon's camera roll
Snapshots from our very good boy on the road.
Where it is
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