What are the opposite words for on the surface?

Antonyms for "on the surface" could be "below the surface," "hidden," or "submerged." These words all suggest that something is not immediately visible or obvious, but requires deeper exploration or investigation. "Below the surface" implies a deeper level of understanding or meaning, while "hidden" suggests intentionally concealing something from view. "Submerged" typically refers to something that is completely covered by water or another liquid. Using antonyms for "on the surface" can add nuance and complexity to your writing or speech, providing a richer understanding of the subject matter at hand.