In a physical sense, "underlying" and "overarching" have very different definitions, but if we use these words in a more abstract way, their meanings seem to intersect.
For example, is there a difference between the phrases: "the underlying theme in this narrative" and "the overarching theme in this narrative"?
I think there is some nuanced difference here, but I can't quite identify it.
Best Answer
Underlying tends to be implicit, because it's underneath--hidden, inaccessible. Overarching tends to be explicit. Everyone can see it. It arches over everything.
Of course, both words also mean "fundamental," so you are right that their meanings intersect.