William Fumarolic gasses (噴気孔ガス, funkikō gasu) are those emitted by a fumarole, a vent into what is mostly groundwater heated to steam by volcanic activity. They are a potential source of energy and are already being utilized throughout Japan, including in Kumamoto, in Oguni and Aso Village (an image of the Oguni plant is below). It seems much potential remains for this free, renewable power source. More can be read (just a bit) about the topic here: https://www.renovainc.com/en/development/minamiaso_yunotani_geothermal/202009_1674/