Corinth Canal on Google Maps.
The Corinth Canal (6.4 km in length and only 24.6 m wide) is an artificial canal that connects the Gulf of Corinth in the Ionian Sea with the Saronic Gulf in the Aegean Sea and separates the Peloponnese from the Greek mainland. Construction of the canal began under Roman Emperor Nerone in 67 AD  but the works ceased shortly after his death. Construction recommenced in 1881 and it has been completed in 1893 but due to the canal's narrowness and periodic closures to repair landslides from its steep walls it was not commercially successful. Today it is a splendid canal traveled by small cruise boats that transport thousands of tourists every year.
Useful links for more informations:
https://corinthcanal.com/?lang=en