depths

A plural with no singular partner: one sinks into the depths, never into the depth. Used bare with the, it means the deep water itself, as when a burning ship goes down into the depths. With of, it names the deepest part of a particular sea, lake or trench. The register is literary, suited to narrative and description rather than to measurement, where the measured plural belongs.

Feelings and mysteries are spoken of here as if they had a floor. The classic collocation is plumb the depths of despair, which borrows the verb for sounding water. One can also be stirred to one's depths, meaning moved at the innermost level. Pair it with nouns of misery, disgrace or obscurity, and expect of to follow: the depths of despair, the depths of a mystery.

The ordinary count plural of depth, used when several measured depths are in play: soundings across a seabed, readings from different decades, the depths of a set of drawers. The same spelling also carries plural-only senses with no singular behind them, such as the literary "deepest part". In those senses, swapping in depth changes or breaks the meaning.

Time takes the place of space: in the depths of winter, in the depths of the crisis. The fixed frame is in the depths of plus a season or ordeal, and it names the harshest stretch. Unlike the "lowest point" sense, which marks a single bottom, this describes a span someone lives through, so it sits naturally after in or during.

Remoteness rather than downwardness: the jungle's depths are its farthest reaches, not its lowest ground. The phrase nearly always runs the depths of plus a place, and it stretches to time as well, so in the depths of the night means far into it. Keep it for evocative writing; it belongs to the same literary, plural-only family as the watery "deepest part".