Water vapor containing hydrogen sulfide and carbon dioxide spews forth at various locations throughout the vicinity of the Unzen Onsen district. With vegetation unable to grow at locations with significant amounts of discharge, a desolate rocky expanse dominates here to form so called Unzen Jigoku ("Hell").
The offensive smell of sulfur fills the air throughout this place. The rocks near fumaroles either degenerate and become brittle or they become caked in sulfur and sinter. In addition to discharged gas, hot-spring water gushes out at many sites and is used to supply water to the Unzen Onsen district.
Since days of old, each individual jigoku site has been named according to details pertinent to its particular history. (Examples of these names include Hachiman Jigoku, Seishichi Jigoku, and Daikyokan Jigoku.) This location is also famous as the site of the persecution of local Christians in medieval times.
Source: Ministry of the Environment