In a dormant manner; in a state of sleep, inactivity, or temporary suspension of function.
From dormant (adjective, from Latin dormire) plus -ly adverbial suffix. This adverb form allows dormancy to be described as an ongoing manner or behavior.
While we use 'dormantly' rarely in modern English, it perfectly captures how volcanoes and diseases can exist 'in a sleeping way'—dangerous but still.
Complete word intelligence in one call. Free tier — 50 lookups/day.