scratch the surface
scratch the surface {v. phr.}
To learn or understand very little about something. — Usually used with a limiting adverb (as "only", "hardly").
We thought we understood Africa but when we made a trip there we found we had only scratched the surface.
High school students have only scratched the surface of their subjects, and even after college graduation, they still find there is much more to learn.
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Source: A Dictionary of American Idioms
