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
What goes around comes around.
Strike the iron while it's hot.
With whom you are friends with, from them you will learn.
If you are chasing two hares at once, you won’t catch a single one.
Don't bring your set of rules in a foreign monastery
When I eat I am deaf and mute.
The quieter you go, the further you'll get.
Better to see once than hear a hundred times.