Turkey is to build a "crazy and magnificent" canal to bypass the tanker-clogged Bosphorus in a massive feat of engineering.
The waterway, to be named Canal Istanbul, would link the Black Sea to the Sea of Marmara, which leads to the Aegean Sea.
It would be around 30 miles long, 80 feet deep and around 500 feet wide.
Prime Minister Recep Erdogan announced the plan during campaigning ahead of elections on June 12.
"We have embarked on the greatest project of the century," Erdogan said, adding that it would be a bigger undertaking than the Panama or Suez canals.
The new waterway would be on the European side of the Bosphorus, he said, but he would not disclose its exact location or the cost of the gargantuan project.
It would be completed by 2023, when Turkey will be celebrating the centenary of the founding of the Turkish republic after the fall of the Ottoman Empire.
"Turkey more than deserves to enter 2023 with such a crazy and magnificent project," he said to a cheering audience in the city. "Istanbul will become a city with two seas passing through it."
Mr Erdogan, who is hoping to win a third term in office in June, had promised to announce what he called a "crazy project" for Istanbul since campaigning began earlier this month, keeping Turks guessing for weeks.
Kemal Kilicdaroglu, leader of the Republican People's Party, brushed off the project saying: "this nation does not need crazy people but people who think".