The scene in which hot water and gas gush out from 250ｍ～300ｍ under ground is called jigoku (hell).
It was formed over 1300 years ago.
There are 8 kinds of Jigoku, like Umi-jigoku, Yama-jigoku, Kamado-jigoku, etc. in Beppu, Oita.
This photo is Umi-jigoku (Ocean Hell).
It is 98℃, but the color is a cool cobalt blue.
Onsen eggs and Jigoku pudding made by the steam here are very popular.
You can also enjoy a warm foot bath.