unfathomable

Adjective

• impossible to come to understand

• resembling an abyss in depth; so deep as to be unmeasurable
  • the abyssal depths of the ocean
    

• of depth; not capable of being sounded or measured

Related words

• Synonyms: 
  abysmal , abyssal
• Antonyms: 
  fathomable , plumbable , soundable