The Great Lakes is/are a major highway for transportation, migration, and trade as well as home to a large number of aquatic species.
I had thought "is" is correct since even though the Great Lakes sounds plural the designation refers to one place and the "a" following is or are indicates that it is a singular (and hence, "is"). However, apparently, the answer is "are", and I'm now very confused. And when I searched Google, it seems that people mostly associate "are" with the Great Lakes. But if I don't know the place beforehand, how can I choose the right answer?