深海不知终老良人不知情深
[shēn hăi bù zhī zhōng lăo liáng rén bù zhī qíng shēn]
The phrase describes a profound but often unreciprocated love in a relationship that goes on without an end, similar to how the depths of the ocean are fathomless and hidden from sight. '良人' here refers to a beloved person, while '不知情深' means not fully understanding the depth of one's emotions.