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
If I knew where I would fall, I would lay some straw there.
(lit.) Whoever gets up early, God gives him
Make a fool pray to God - he will break his forehead.
Poor people are inventive.
Crowded, but not aggrieved.
Better late than never.
If you like sledding, you should like to carry the sled.
What goes around comes around.