в тихом омуте черти водятся

Etymology
lit. 'in a quiet pool, devils dwell.'

Proverb

 * 1) beneath a placid exterior, a person may harbor turbulent desires.

Usage notes

 * Frequently mistranslated as "still waters run deep." The English expression means that someone has hidden, often positive traits. The Russian expression is predominantly used negatively, meaning a deceptively calm person may hide harmful traits.