Despite the mountain’s silence, it isn’t difficult to imagine the thundering crush of the Earth exploding through itself on Mt. Adatara. An active volcano in Fukushima Prefecture, Mt. Adatara last vented its anger in 1996. An eruption almost a century prior ended the lives of 72 local mine workers. A beautiful sight it may be, but Mt. Adatara is for real.

Bizarrely, if you can face the dangers of this extreme landscape, you can find deep relaxation. At 1,700-meters in height, Mt. Adatara is not only home to geological frustrations, but also one of Japan’s wildest delights. On the western slope of the mountain runs the Iogawa, or Io River. At its headwater, it may seem a regular stream of fresh mountain water; but the river eventually carves through a landscape of sulfurous valley fumes, steaming hot waters, and the milky pools of Japan’s most desolate hot spring experience.

Lorded over by a volcanic mountain scarred with fumarole vents belching out primordial heat, the natural spring waters of Numajiri Yumoto Onsen gush out at over 10,000 liters per minute. At a scalding 80 degrees Celsius, Japan’s “extreme onsen” is no joke. While there are plenty of diluted ponds, streams and rivulets safe to bathe in, there are no actual baths. Nor are there facilities of any kind. Only pools of hot water and Fukushima’s rugged mountainscape. A wilder relaxation is hard to imagine.

Numajiri Yumoto’s remote location means that keeping its trail in tip-top hiking condition is not the easiest of tasks. At times overgrown with plants and featuring plenty of loose rock, the two-hour round trek is a respectable study in concentration. Heavy snowfall during winter means the climb is simply not possible from December until April. Having said that, one needn’t be a professional mountaineer to enjoy the challenge, though it is imperative to bring along decent footwear, appropriate clothing and no small degree of common sense.

The trail along the forested base of the mountain features a beautiful bonus during the fall season. It is during this time that the colors of Japan’s famous autumn leaves exert their kaleidoscopic charm. An added surprise along the trail are the peering views of the 150-meter-high Shiraito Falls. Surrounded by such a lush environment, one can hardly imagine the lunar landscape that lies ahead.

The trail eventually heads back down into what can only be described as a blasted valley gripped by the hand of hell. And yet, isolated and remote, the valley’s beauty cannot be ignored. Fringed with brave green shrubs, the hypnotic lure of steaming vats of mountain water seared with geothermal intensity gives this place an otherworldly appeal.

During the cooler months of spring and fall (winter visits are not possible), the heat of the water sends tufts of steaming delight into the frigid air—as clear an invitation to soak if there ever was one. Wooden pipes follow the river, siphoning off its geothermal pleasure and transporting it to the region’s many hotels, filling their hot springs with the same healing warmth—but without the danger.

If it hasn’t been stressed enough, be careful when bathing here. The waters themselves have a pH level hovering around two. What does that mean? Consider bathing in hot lemon juice, therapeutic and invigorating, to be sure, but any hidden cuts or wounds on your skin will soon cry out for attention. Then there is the sulfurous smell of rotten eggs—the deeper into the mountain, the stronger it becomes.

Rocky and steep though the trail may be, it is those leaking sulfurous gases that are the real danger. In some sections of the trail, the valley forms up into bowl-like formations. Here, the build up of fumes can be lethal. As a result, if you plan a trip here you must hire a local guide. Equipped with gas monitors and first-aid kits, the guides understand the mountain,
know its signs, and importantly, know what to do in an emergency.

Once it’s safe to proceed—and it usually is—it’s time to take full advantage of Japan’s most extreme secret pleasure. Swimwear is mandatory within a mixed- gender tour, and the signs warning of the gas lurking ahead beyond the pools must be respected. Otherwise, few things can compare to relaxing in a pool of geothermal grandeur. Mt. Adatara might come with some nerve-wracking moments, but the Numajiri Yumoto “extreme hot spring” is an experience like no other.

Numajiri Yamoto Onsen

Where: 2864 Inawashiro-cho, Yama-District, Fukushima 969-2752

Getting there: Take the Tohoku Shinkansen from Tokyo, and transfer at Koriyama Station to a train bound for Aizuwakamatsu. Get off at Inawashiro Station for a 25-minute taxi ride to Cafe & Activity Nowhere.

How much: The hot springs are free, but a guide (required) will cost ¥24,200 for a single person. Prices are lower prices for groups up to eight people.

When: May–November, depending on weather conditions

Hike times: Friday–Monday, 9–12pm and 1–4pm

Website: numajiri-lodge.com/nowhere/ (Japanese)

Tips: Try hiking nearby Mt. Bandai the day before or after

Good for: Solo adults, couples and small groups