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
A bad example is contagious.
The demon is not as scary as he is depicted.
Оne sees the mote in another's eye and ignores the log in his own.
Without labor, you can not even catch a fish in the pond.
A titmouse in hands is better than a crane in the sky.
If you like sledding, you should like to carry the sled.
Do not dig a hole for another one, you will fall into it yourself.
You can't spoil the porridge with butter.