The 'Great Boiling Valley' was created 3000 years ago when Kami-yama erupted and collapsed, also forming Ashino-ko. Hydrogen sulphide steams from the yellow ground here (the yellow is crystallised sulphur) and the hot water is used to boil onsen tamago, eggs blackened in the sulphurous waters, which you can buy to eat from the tourist shops beside the ropeway station. On rare occasions Ōwakudani might be closed due to volcanic activity.
The one-hour Ōwakudani-Tōgendai Nature Trail has been mostly closed since 2015 due to the presence of toxic gases.