Komatsu Jigoku is in the Aso Kuju National Park. Here, one can see erupting hot mud. It is said that Komatsu Jigoku got its name jigoku (hell) from the red rocks and steam that are created as a result of volcano activity. There is a walking path that people can walk around the area in about 20 minutes (*straying away from the walking path is dangerous so please be careful). The walking path is warm because of the geothermal heat and steam, and the area smells like sulfur because of the hot springs. 
Nearby the track, there is Hatchobaru Geothermal Power Plant that utilizes geothermal power. There is also Sujiyu Onsen where you can steam using hot spring steam if you bring eggs with you. You put raw eggs in a basket attached to a long stick and you can make hot spring eggs in about 9 minutes. A famous dish in Sujiyu Onsen is Gokuraku hot chicken steamed wildly using hell fumarole. It uses fresh free-range chicken stuffed with cabbage and onions.