The Extreme Onsen: Nakanosawa-numajiri Onsen
Possibly the greatest wild onsen in Japan, or the world? This massive onsen river in the mountains is the largest of its kind in all of Japan! With the help of a professional guide, visitors can traverse unique volcanic terrain to reach this extreme onsen river in the mountains. Bathing in the water here is thought to have many health benefits, as well. Read more below for additional information about this experience!
Poisonous Volcanic Gasses and Safety (IMPORTANT!)
First of all: SAFETY.
Completing this hike takes approximately two-hours roundtrip, it has several stretches of difficult and potentially dangerous terrain. The main danger is that this mountain is a volcano, so, it is constantly releasing poisonous gasses. These gasses sometimes accumulate to dangerous and deadly levels that can cause fainting and even death. Fortunately, professional guides are available and trained with the tools necessary to safely guide you on your journey.
If you are interested in doing this hike, tours can be booked through our tours page.
The hike is beautiful, with views of a massive waterfall and the surrounding mountains.
Tunnels of trees reminded me of the entry way to a mysterious world. As you get higher, the trail slopes downwards on either side so that there are panoramic views of the surrounding area. Suddenly, you can see the terrain has changed up ahead from green forest to white and red volcanic stones. This way once the setting of a violent volcanic eruption, and the thought of that feels outlandish as the mountain is peacefully quiet.
Descending into the volcanic valley
The trail drops steeply into the valley, where shadows preserve small pockets of winter snow well into the spring months, something that is important to consider if you are visiting in spring. (Vising in winter would be extremely dangerous and is therefore prohibited.)
As you continue, the trail can be difficult to identify due to plant overgrowth, the remote nature of this trail and onsen can make it difficult to keep the path clear. When I visited, I was grateful for my guide who kindly helped me cross the large pockets of snow and ice as well as the sections where bamboo shoots had encroached on the trail, making it difficult to pass.
As you descend deeper into the valley, you can appreciate the way the valley forms a bowl of reddish volcanic stone and soil. Unfortunately, this unique shape is what can contribute to the accumulation of fatal levels of poisonous gas! Our guide tested the air and conditions, and determined that we were safe to explore.
A river of warmth
The blue river of onsen water contrasts sharply with the warm tones of the volcanic landscape. It felt like we had discovered water on mars. Steam rose from the water and it was amazing how warm the water stayed despite being so exposed to the cool spring air. Wooden channels split off from the river, this onsen water will flow through the wooden channels, to underground pipes and fill the baths at eleven different onsen hotels where it can be enjoyed by guests who want to experience the health benefits of this onsen water without the need to go hiking.
Bathing in the onsen water is thought to have medicinal benefits. The water has a pH of 2.1 that is comparable to lemons! It is unique in Japan as the largest amount of hot spring water to come from one source, the “Numajiri Motoyu,” which is inaccessible to humans. So if you choose to visit, I hope you will bathe in the water here and experience the refreshing effects of this onsen!
We will not provide the exact trail information for this hike due to the dangerous nature of poisonous volcanic gasses in the area which have been fatal to some hikers. You may find information about the trail online, these the sources reference a different version of the trail that is illegal, and crosses over protected land. In order to experience this beautiful and unique environment in a safe and respectful way, we encourage visitors to hire guides or visit as a part of tours that include guides. Unfortunately, some have chosen not to hire a guide, resulting in a number of casualties on the mountain. Please help us to avoid further tragedies and do not attempt this hike without an experienced guide or encourage others to do so. Thank you for your cooperation.
Extreme Onsen Experience Tour is available from here!