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
When I eat I am deaf and mute.
Beast runs towards the hunter.
The quieter you go, the further you'll get.
You can't help grief with tears.
A titmouse in hands is better than a crane in the sky.
Where there is no fish even a crayfish is a fish.
Patience and labor will grind everything.
Make a fool pray to God - he will break his forehead.