This is the 11070th most frequent Russian word.
глубокой
Deep.
Here, 'глубокой' as 'deep' metaphorically describes the intensity of one's state of thought or reflection.
Она сидела в глубокой задумчивости.
She sat in deep thought.
In this case, 'глубокой' refers to the physical property of 'deep' as applied to the lake's depth.
Озеро было глубокой и необъятной водной гладью.
The lake was a deep and vast expanse of water.
Here, 'глубокой' describes 'darkness,' emphasizing its intensity and visual obscurity.