Which of the following best describes the opposite of "superficial"?

The term "superficial" refers to something that is shallow or lacking in depth, whether in a physical or metaphorical sense. When considering the opposite of "superficial," the word "deep" encapsulates the idea of being profound or having substantial depth, both literally and figuratively.

In contexts where "superficial" might describe someone's understanding or an analysis that lacks thoroughness or insight, "deep" is aptly used to convey the opposite quality—signifying a strong, thorough, and careful examination of a subject. Thus, "deep" effectively serves as the antonym for "superficial," highlighting a richness and complexity that is absent in superficial observations.