Hidden depths phrase
Used to refer to complicated aspects that are not easily recognized or visible.
His performance revealed his hidden depths.
She uses her sense of humor to hide her hidden depths.
Used to refer to a hidden place or thing that needs a lot of searching or prior knowledge.
This new subject has lots of hidden depths that I need to explore.
Meaning absolutely certain or without a doubt
I am as sure as God made little green apples that Peter's the one for you, you two make a beautiful couple.