Still waters run deep

Meaning: Some rivers have rough surfaces with waves. That's usually because the water is shallow and there are rocks near the surface. But deep rivers have no rocks near the surface and the water is smooth and still. "Still waters run deep" means that people who are calm and tranquil on the outside, often have a strong, "deep" personality.

Note: still (adjective) = calm, motionless | deep (adjective) = going far down