Location
Depictions of Opal in the Starman series showed it as a generally Eastern-US city with a port (though not necessarily a seacoast) and being surrounded by generally flat terrain. Recent issues of the current JLA series, specifically the "Crisis of Conscience" arc, have explicitly established Opal City as being located in Maryland, per Robinson's longstanding opinions on the subject in various interviews with fans.
Read more about this topic: Opal City