Meaning of the word "still waters run deep" in English

What does "still waters run deep" mean in English? Explore the meaning, pronunciation, and specific usage of this word with Lingoland.

still waters run deep

US /stɪl ˈwɔː.tərz rʌn diːp/
UK /stɪl ˈwɔː.təz rʌn diːp/
"still waters run deep" picture

Idiom

a person who seems quiet or shy may have a very complex, intelligent, or passionate internal life

Example:
He doesn't say much, but still waters run deep.
I never knew she was so talented; I guess still waters run deep.