The phrase "in the depths" is often used to describe something that is very deep or far below the surface. There are a few synonyms that can be used to convey a similar meaning. "Down in the abyss" or "deep within the trenches" can be used if something is very deep and hard to reach. For something that is difficult to understand, "in the bowels" or "in the heart" can be used instead of "in the depths." "Submerged in" or "buried beneath" are other good options for describing something that is hidden or covered up. Overall, there are many ways to convey the meaning of "in the depths" through synonyms and related phrases.