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