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

Remaining hopeful during times of tragedy Sharabi said finding light during unfathomable darkness is what has gotten him through in the days following the tragedy in Australia. —Anna McAllister, CBS News, 16 Dec. 2025

Rob and Michele's murders have left the entertainment world reeling from an unfathomable tragedy. —Lauryn Overhultz, FOXNews.com, 15 Dec. 2025

But the supermassive black hole lurking at the core of NGC 3783 is 30 million times the mass of our humble sun, and the magnetic field writhing around is millions of times stronger, so when its lines snap and reconnect, the resulting flare is an eruption of almost unfathomable power. —Kiona N. Smith, Space.com, 9 Dec. 2025

The film captures how the siblings came together to gain closure after an unfathomable loss. —Liam Quinn, PEOPLE, 5 Dec. 2025

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