Kusatsu Onsen is located at the foot of "Kusatsu-Shiranesan". Kusatsu-Shiranesan is one of the 111 active volcanoes in Japan, and active volcanic activity is quietly occurring underground. The good quality and abundance of Kusatsu Onsen are the blessings of a living volcano.

Kusatsu-Shiranesan has a crater lake called Yugama. Its beautiful colors are created by hot spring water and volcanic gases that come up from the bottom of the crater. The volcano delights visitors with its beautiful scenery.

How Kusatsu Onsen works:
Kusatsu-Shiranesan has four floors. The key point is that each floor has a different flow rate of water. Rainwater seeps into the ground and becomes groundwater, which is mixed with volcanic gases. The hot spring water thus created flows slowly through the ground, mixing with the rocks and adding various components. The water then gushes out from the plateau at the foot of the mountain at just the right temperature.

Rainwater soaks into the new lava (which is scoured with many cracks) and flows quickly to form spring water.
Older lava has fewer fractures but a larger volume, so a lot of groundwater flows slowly. Here, volcanic gases mix with the water to form hot spring water.
The old pyroclastic plateaus are difficult for water to pass through, preventing the hot spring water from the old lava flows from seeping deeply into the ground. As a result, hot spring water gushes out from the top of old lava flows.
If there is too much magma gas, it turns into white smoke, and if there is too little, it turns into a cold mineral spring. In other words, Kusatsu Onsen has a miraculous balance between the ratio of magma gas and rainwater.

*Caution
As of 2024, due to restrictions on entry to the Kusatsu-Shiranesan area, it is not possible to visit the yugama.

Kusatsu-Shiranesan Official/Kusatsu Onsen Portal
https://www.kusatsu-onsen.ne.jp/category/detail/index.php?c=2&g=2&kcd=29


Google MAP
https://maps.app.goo.gl/Kpr7GjuBy6AXiAyT7?g_st=il

Credit: Tokyo Institute of Technology Kusatsu-Shirane Volcano Observatory
Cooperation: Kusatsu-Shirane Volcano Observatory, Tokyo Institute of Technology

Kusatsu-Shiranesan has frequently caused disasters due to eruptions. The Kusatsu-Shirane Volcano Observatory of the Tokyo Institute of Technology is located in Kusatsu Town, where volcanic research is actively conducted. The results of this research are used for volcanic disaster prevention measures and management of hot spring resources in Kusatsu Town.