Arthur Schopenhauer Quote

No one knows what capacities for doing and suffering he has in himself, until something comes to rouse them to activity: just as in a pond of still water, lying there like a mirror, there is no sign of the roar and thunder with which it can leap from the precipice, and yet remain what it is; or again, rise high in the air as a fountain. When water is as cold as ice, you can have no idea of the latent warmth contained in it.

Studies in pessimism. A series of essays... Selected and translated by T. Bailey Saunders... Third edition (ed. 1892)