The Mediterranean is a region of land on Earth encompassing southern Europe and northern Africa, as well as the body of water separating the two shores.
The aliens who portrayed themselves as the Greek gods settled in this region, specifically in what would later be known as Greece, during the 28th century BCE. (TOS: "Who Mourns for Adonais?")
Apocrypha
In the novelization of Star Trek: The Motion Picture a political unit located in the region, known as the "Mediterranean Alliance", encompassed at least one former nation-state, Gibraltar, during the 2270s. Admiral Kirk was contacted by Starfleet during an excursion to Gibraltar, which is noted to be a member of this alliance.