unfathomable

adjective

un·​fath·​om·​able ˌən-ˈfa-t͟hə-mə-bəl 

: not capable of being fathomed:

a

: immeasurable

… the unfathomable darkness of empty space.

—H. G. Wells

b

: impossible to comprehend

unfathomable motives/reasons

unfathomably

ˌən-ˈfa-t͟hə-mə-blē 

adverb

Synonyms of unfathomable

• infinite
• endless

Examples of unfathomable in a Sentence

His behavior is completely unfathomable.

the unfathomable reaches of space

Recent Examples on the Web

The movie explores a few unique forms that extraterrestrial life could take, from space microorganisms that produce both infrared light and an unfathomable amount of energy, to rocklike aliens that live under crushing pressure and breathe ammonia. —Mary Magnuson, The Conversation, 20 Mar. 2026

Like a flower that somehow emerges through concrete, love can blossom even in a space of unfathomable violence. —Matthew Carey, Deadline, 19 Mar. 2026

For reasons this review cannot reveal, the Clyburns of New York find themselves in Montana to confront an unfathomable accident and all the gut-wrenching emotions and paperwork that come amid loss. —Aramide Tinubu, Variety, 13 Mar. 2026

But give the man credit for putting up numbers that are borderline unfathomable, small sample size notwithstanding. —Barry Jackson, Miami Herald, 10 Mar. 2026

Word History

First Known Use

1640, in the meaning defined above

Kids Definition

unfathomable

adjective

un·​fath·​om·​able ˌən-ˈfat͟h-ə-mə-bəl 

: not capable of being fathomed:

a

: immeasurable

b

: impossible to understand