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 industry still cannot come to grips with the previously unfathomable scenario of the strait staying shuttered for a prolonged period of time. —Jordan Blum, Fortune, 28 Mar. 2026

An unfathomable 6,249 career points. —Joseph Dycus, Mercury News, 28 Mar. 2026

Last year, Justin Northwest prodigy Cooper Lutkenhaus and San Antonio Harlan wunderkind Tate Taylor set high school national records with incredible times that once seemed unfathomable. —Greg Riddle, Dallas Morning News, 27 Mar. 2026

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

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