The 'Great Boiling Valley' was created 3000 years ago when Kami-yama erupted and collapsed, also forming Ashino-ko. Hydrogen sulfide steams from the ground here and the hot water is used to boil onsen tamago , eggs blackened in the sulphurous waters, which you can buy to eat (they're fine inside).
The Ōwakudani-Tōgendai Nature Trail, is a one-hour hike you can do here but don't linger if the toxic gases are strong as they are dangerous.
