bottom depths of water; abyss
Refers to the deepest part of a body of water, often with a literary or dramatic tone.
Metaphorical extension: the deepest, most hidden part of something non-physical, such as despair or a mystery.
Archaic adverbial use meaning 'to the very bottom' or 'thoroughly'; not used in modern Japanese.
It is said that ancient ruins lie at the bottom depths of that lake.
He was sunk in the depths of despair.
In classical texts, it was used as in 'to know thoroughly'.