With accent marks:
В ти́хом о́муте че́рти во́дятся.
In a quiet whirlpool, demons live.
This proverb usually characterizes an unremarkable, inconspicuous person who suddenly turned out to be capable of extraordinary acts, not always good.
The proverb comes from the old Slavic beliefs, according to which the whirlpools are the habitat of water demons and evil spirits. It was believed that they were the ones who dragged swimmers to the bottom.
The English analog of this proverb is "Still waters run deep".
Other Russian proverbs
Where it is thin, it breaks.
Fear has big eyes.
With a darling, even a hut is a paradise.
The best is the enemy of the good.
What you will sow, that you will reap.
Another's soul is darkness.
A titmouse in hands is better than a crane in the sky.
Start taking care of the dress while it's new, and of your honor while you are young.