different between unfathomable vs fathomless

English

un- +? fathom +? -able.

unfathomable (comparative more unfathomable, superlative most unfathomable)

Impossible to fathom or understand.
Synonyms: fathomless, incomprehensible, unintelligible; see also Thesaurus:incomprehensible
Antonyms: explicable, intelligible; see also Thesaurus:comprehensible
Difficult to penetrate.
Synonyms: impenetrable, impregnable
Antonyms: penetrable, pregnable

fathom +? -less

fathomless (comparative more fathomless, superlative most fathomless)

Very deep (especially of water deeper than a lead line can measure); bottomless.
(by extension) unfathomable or incomprehensible.

(very deep): soundless; See also Thesaurus:deep
(incomprehensible): ungraspable, unknowable; See also Thesaurus:incomprehensible

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