Japan is one of the most volcanically active countries in the world and as a result, contains an abundance of natural hot springs.
Nihongi No Yu
Nihongi no yu, located at the base of the Jizo Pass, is known for its brownish-red carbonated waters. There is also a small shop inside and soba noodles on offer. On a clear day, bathers can also enjoy views of the surrounding mountains.
On a nice day, you will have a view of Mt. Komagatake, Mt. Mugikusa, Mt. Misawa.
There is a Japanese-style room where you can taste their soba, "Gohei-mochi", mountain vegetables after bath. They also sell signature products, vegetables too.
The bath is not too hot but you will become warm from inside.
It is well known among health-conscious people and those who play sports.
However, there is not much natural soda hot springs suitable to bathe so it is quite rare.
http://www.nihongi-spa.com/
Seseragi no toki
Seseragi no Toki onsen is famous for its rust-coloured carbonated water which is said to give smoother and more radiant skin. It is located on the Kaida Plateau, a 5 minute drive from Kiso-Fukushima.
Komorebi no yu
Komorebi no yu uses water flowing directly from the hot spring source. Bathers can watch the light filtering through the trees while enjoying a long soak in the sodium and calcium bicarbonate spring water.