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
Appetite comes with eating.
In a healthy body, healthy mind.
Without labor, you can not even catch a fish in the pond.
The morning is wiser than the evening.
Eggs don't teach the chicken.
Better to see once than hear a hundred times.
A bad example is contagious.
Better late than never.