unfathomable

un- +‎ fathom +‎ -able.

unfathomable (comparative more unfathomable, superlative most unfathomable)

1. Impossible to fathom or understand.

   Synonyms: fathomless, incomprehensible, unintelligible; see also Thesaurus:incomprehensible

   Antonyms: explicable, intelligible; see also Thesaurus:comprehensible

   The sheer numbers they attacked with were unfathomable.

   • 2013 April 9, Andrei Lankov, “Stay Cool. Call North Korea’s Bluff.”, in New York Times‎:

     Suggestions that those leaders are irrational and their decisions unfathomable are remarkably shallow. North Korea is not a theocracy led by zealots who preach the rewards of the afterlife.

2. Difficult to penetrate.

   Synonyms: impenetrable, impregnable

   Antonyms: penetrable, pregnable

• unfathomability
• unfathomableness

• unfathomed
• unimaginable
• inconceivable