unfathomed

Adjective

• situated at or extending to great depth; too deep to have been sounded or plumbed
  • the profound depths of the sea
    
  • the dark unfathomed caves of ocean-Thomas Gray
    
  • unplumbed depths of the sea
    
  • remote and unsounded caverns
    

Related words

• Synonyms: 
  profound , unplumbed , unsounded