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From Gifu! Hot Spring Museum Episode 4: Natural Hot Springs Erupting Naturally

A hot spring that naturally erupts to the surface is called a natural erupting spring source. Nowadays, many hot springs are obtained by drilling deep underground, and there are limited places where you can see hot springs naturally erupting.
As limited places where you can still witness hot springs naturally erupting,

1. Places called "Hell Valley" or "Hell" in volcanic areas
2. Large-scale spring sources that are carefully protected as symbols of hot spring towns
3. Spring sources that have been erupting directly from the bottom of bathtubs in hot spring inns for a long time

are mentioned.

(1) Natural Erupting Spring Sources in "Hell" Terrain

In a corner of the "Hell" terrain of Tamagawa Onsen in Akita Prefecture, there is the "Dai Fun" spring source, boasting Japan's largest eruption volume of 9,000 liters per minute (equivalent to 45 drums). Recent studies have reported data exceeding 10,000 liters per minute. You will be overwhelmed by the sight of 98°C hot springs gushing out from large eruption holes. The "Hell" area of Tamagawa Onsen is equipped with a nature exploration path, allowing you to observe various forms of hot spring eruption phenomena while enjoying a walk.
Other natural erupting spring sources seen in places called "Hell" in volcanic areas include Noboribetsu Onsen in Hokkaido, Goshogake Onsen and Doroyu Onsen in Akita Prefecture, Katayama Hell in Miyagi Prefecture, Tateyama Hell Valley in Toyama Prefecture, and Unzen Onsen in Nagasaki Prefecture.

The Great Eruption Source of Tamagawa Onsen, Akita Prefecture	The Spring Source of Goshogake Onsen, Akita Prefecture

As mentioned before, hot springs and volcanoes are closely related, especially in the case of volcanic hot springs, where the two cannot be discussed separately. In areas where volcanic activity is relatively active, you may see fumarolic areas emitting white smoke of volcanic gas or steam not only at the top of the volcano but everywhere. This area is transformed into a desolate landscape with white rock surfaces exposed due to the influence of geothermal heat and hot water (hot spring water) containing various components, turning rocks into clay minerals called hot spring residual soil. Such places are called hydrothermal alteration zones.
In hydrothermal alteration zones, high-temperature hot springs may erupt in large quantities from everywhere. The sight of high-temperature hot springs naturally erupting in a desolate situation led people in the past to associate the image with the landscape of hell in the afterlife, and in various parts of Japan, such terrains with hot spring phenomena are named "Hell." The large amount of hot springs erupted from "Hell" eventually erodes the weathered and fragile rock surfaces, forming valleys. Many valley-shaped terrains with hot spring phenomena formed in hydrothermal alteration zones are called "Hell Valley." The former names "Hell" and "Hell Valley" are thought to have been for objects of faith, but now they are used as words to describe the terrain of hydrothermal alteration zones with hot spring phenomena. In Gifu Prefecture, a part of the Hell Valley in Shirakawa Village, Ohshirakawa, shows a hydrothermal alteration zone with fumaroles.
The following are known as major "Hell Valley" terrains.
Mount Iou (Hokkaido), Noboribetsu Onsen (Hokkaido), Osorezan (Aomori Prefecture), Kawarage Hell (Akita Prefecture), Doroyu Onsen (Akita Prefecture), Goshogake Onsen (Akita Prefecture), Tamagawa Onsen (Akita Prefecture), Fukenoyu Onsen (Akita Prefecture), Katayama Hell (Miyagi Prefecture), Shiobara Onsen Shinyu (Tochigi Prefecture), Kusatsu Onsen (Gunma Prefecture), Manza Onsen (Gunma Prefecture), Owakudani Onsen (Kanagawa Prefecture), Tateyama Hell Valley (Toyama Prefecture), Unzen Onsen (Nagasaki Prefecture), Komatsu Hell (Oita Prefecture), etc.
All of them are hot spring areas with abundant hot water having large spring sources in Hell Valley. Among them, the Hell Valley (fumarolic area) of Goshogake Onsen and Tamagawa Onsen is equipped as a nature research path, allowing you to observe the diverse nature of Hell Valley.
"Hell Valley" is an environment where plants find it very difficult to grow due to the influence of geothermal heat and volcanic gases containing hydrogen sulfide. In such places, a few plants called sulfur vent plants, such as Empetrum nigrum and Gaultheria miqueliana, which can withstand the environment of hot spring areas, grow.

The Fumarolic Area of Ohshirakawa Hell Valley	The Hell Valley of Noboribetsu Onsen, Hokkaido
The Hydrothermal Alteration Zone of Kawarage Hell, Akita Prefecture	Tateyama Hell Valley, Toyama Prefecture

(2) Natural Erupting Spring Sources as Symbols of Hot Spring Towns

On the other hand, there are spring sources that are carefully protected as symbols of hot spring towns. Some hot spring towns have developed around these spring sources, and natural erupting spring sources are deeply connected to the life of hot spring areas.
Symbolic natural erupting spring sources in hot spring areas include "Yubatake" in Kusatsu Onsen, Gunma Prefecture, "Ogama" in Nozawa Onsen, Nagano Prefecture, "Arayu" in Yumura Onsen, Hyogo Prefecture, and "Tsuboyu" in Yunomine Onsen, Wakayama Prefecture. In Shirako Onsen, Gifu Prefecture, there is a spring source pond where hot springs naturally erupt, and the source is drawn to inns from there.

The Spring Source Pond of Shirako Onsen	The "Yubatake" Spring Source of Kusatsu Onsen, Gunma Prefecture
The "Arayu" Spring Source of Yumura Onsen, Hyogo Prefecture

(3) Spring Sources Erupting Directly from the Bottom of Bathtubs

In spring sources that erupt at a suitable temperature for bathing, there are places where bathtubs are built above them, allowing hot springs to erupt directly from the bottom of the bathtubs. This provides the advantage of bathing in the freshest spring source that has just erupted.
You can enjoy bathing in spring sources erupting directly from the bottom of bathtubs at places like Yakushi Onsen in Niseko, Hokkaido, Tsuta Onsen and Yachi Onsen in Aomori Prefecture, Namari Onsen in Iwate Prefecture, Tsurunoyu in Nyuto Onsenkyo, Akita Prefecture, Tokusa Onsen in Fukushima Prefecture, Hoshi Onsen in Gunma Prefecture, Masutomi Radium Onsen and Shimobe Onsen in Yamanashi Prefecture, Shibu Goten-yu in Oku-Tateshina Onsen, Nagano Prefecture, Tsuboyu in Yunomine Onsen, Wakayama Prefecture, Maga Onsen in Okayama Prefecture, Chihara Onsen in Shimane Prefecture, and Kawasoko Onsen in Oita Prefecture.

The Bathtub with Direct Spring Source of Shibu Goten-yu in Oku-Tateshina Onsen, Nagano Prefecture	The Bathtub with Direct Spring Source of Chihara Onsen, Shimane Prefecture