The Isthmus of Corinth is the 6.3 km wide strip of land linking the Peloponnesus to the continent. A portage road (diolkos) was built in the 7th century BC as a way to transport cargo and war ships across the isthmus. The ships were thus spared the long and dangerous route around the mass of the Peloponnesus.
Historians do not agree on the number of people needed to drag a ship into the large carts and on the trail. From 100 to 200 men for a trireme seems to be the right range. Perhaps galley crews pulled their own ships (a trireme had about 170 rowers). The trip would take about three hours.
Demetrius of Macedon would try to have a canal built, but never excavated. His surveyors overestimated the difference in levels between the two seas to be linked by the canal and feared catastrophic inundations would ensue. A modern canal was finally finished in 1893.