Completed in 1893, the Corinthian Canal is straight as a ray of light. It is 4 miles long, and 70 feet wide. None of those ships have been photoshopped in. Their sides may have been greased, though.
It makes the Peloponnesian Peninsula a technical island, connecting the Gulf of Corinth and the Saronic Gulf in the Aegean. Caesar, Caligula, and Nero all tried to dig it, and all died violently before making progress. Some of Nero's work can still be seen.