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.
Repetition is the mother of learning.
Don't go to the woods if afraid of wolves.
Do not look at a given horse's teeth.
You can't spoil the porridge with butter.
A well-fed man does not understand a starving one.
The apple never falls far from the tree.
Fear has big eyes.