The famous Greek geographer Strabo wrote in the first century A.D., that Piraeus was formerly an island and lay ‘over against’ the mainland, from which it got its name. To validate Strabo's hypothesis, cartographic and historical data were compiled with multiproxy paleoenvironmental analyses and radiocarbon dating from a series of boreholes drilled in the Cephissus coastal plain, southwest of Athens, Greece. The results of this interdisciplinary geoarchaeological research demonstrate the reliability of Strabo's text by revealing that Piraeus was indeed an island. In early Holocene time, the rocky hill of Piraeus was linked to the mainland of Attica. During the late to final Neolithic Period (4850–3450 B.C.), Piraeus became an island in a shallow marine bay, due to sea-level rise in the Holocene. Between 2850 and 1550 B.C., in the Early and Middle Bronze Age, Piraeus was separated from the mainland by a wide lagoon. In the fifth century B.C., Themistocles, Cimon, and then Pericles connected Athens to Piraeus by building two “long walls” partly built on a residual coastal marsh called the Halipedon. This study reveals an impressive example of past landscape evolution.