The Isthmus of Corinth is the narrow land bridge which connects the Peloponnese peninsula with the rest of the mainland of Greece, near the city of Corinth. The Isthmus was known in the ancient world as the landmark separating the Peloponnese from mainland Greece. In the first century AD the geographer Strabo noted a stele on the Isthmus of Corinth, which bore two inscriptions. One towards the East, i.e. towards Megara, reading: "Here is not Peloponnesus, but Ionia" and the one towards the West, i.e. towards the Peloponnese: "Here is Peloponnesus, not Ionia" ; Plutarch ascribed the erection of the stele to the Attic hero Theseus, on his way to Athens.

The Diolkos was a paved trackway near Corinth in Ancient Greece which enabled boats to be moved overland across the Isthmus of Corinth. The shortcut allowed ancient vessels to avoid the long and dangerous circumnavigation of the Peloponnese peninsula. The phrase "as fast as a Corinthian", penned by the comic playwright Aristophanes, indicates that the trackway was common knowledge and had acquired a reputation for swiftness.