你心如深海深不见底
[nĭ xīn rú shēn hăi shēn bù jiàn dĭ]
'Your heart is as profound as deep sea' means the user thinks the other’s heart or mind is difficult to understand or reach the depths just like it's tough to fathom the abyssal parts of oceans. Expresses mystery, complexity in someone else's nature or intentions.