invisibly       
adverb uk/ɪnˈvɪz.ə.bli/ us/ɪnˈvɪz.ə.bli/
 看不见地,隐形般地
in a way that is impossible to see, or that is not noticed
  This crisis has been building invisibly for years. 多年来,这场危机一直在无形中加剧。 
 They swam almost invisibly in the shadows. 他们游在阴影中,几乎完全看不见。 
 See: invisible
 - More examples
 - They had servants who almost invisibly cleaned up after them.
 - Some cracks begin invisibly inside the wheels.
 - She lived quietly and almost invisibly in the United States.