an artificial canal in Greece that connects the Gulf of Corinth in the Ionian Sea with the Saronic Gulf in the Aegean Sea
It is 6.4 kilometres (4 miles) in length and only 24.6 metres (80.7 feet) wide at sea level, making it impassable for many modern ships
The Corinth canal concept originated with Periander of Corinth in the 7th century BC. Daunted by its enormity, he chose to implement the Diolkos, a land trackway for transporting ships, instead
Construction of a canal finally began under Roman Emperor Nero in 67 AD
Read more here https://en.wikipedia.org/wiki/Corinth_Canal
So it only became a thing somewhat recently.
It connects the Aegean Sea with the Ionian sea and it bypasses the Peloponnese peninsula, which is a relatively significant landmass. I guess the cost and the time/fuel savings might not be significant enough to modernize it. I wouldnt be surprised if they make it wider in the future.