Fetching details from the dictionary...
dormant volcano
その山は長年噴火していない休火山だ。
That mountain is a dormant volcano that hasn't erupted for years.
休火山が突然噴火する可能性もある。
There is also a possibility that a dormant volcano could suddenly erupt.
休火山の火口湖は観光名所になっている。
The crater lake of the dormant volcano is a tourist attraction.