This video, titled "Amazing as Expected! The Natural Hot Spring of the Best Unexplored Region of Kanto. Nasu Nakanosawa Hot Spring" (やっぱりすごかった！関東最強秘境の野湯。那須中ノ沢温泉), was released by "Suu San's Outdoor Activities" (すーさんの外遊び outdoor activities).

There are a number of hot springs in Japan that are called "hidden hot springs," but Nasu Nakanosawa Onsen in the Kanto area is the one that attracts attention from hot spring fanatics.
Nasu Nakanosawa hot spring is located in the Nasu area of Tochigi Prefecture, near the border of Fukushima Prefecture, and is located at the end of a mountain path full of bushes.
For this hot spring found in nature, you can dig around yourself to shape the hot springs and adjust the temperature of the hot springs to your liking.
The road to the hot spring is quite steep, but after all the hard work, the ultimate hot spring awaits.