Turkey has for the first time connected its European and Asian sides by opening a railway tunnel that fulfils a vision first proposed by an Ottoman sultan about 150 years ago.
The tunnel is 13.6km long, with 1.4km running under the Bosporus, the strait that connects the Black Sea to the Sea of Marmara, and divides Istanbul between Asia and Europe.
Turkey will celebrate two feasts together. We will mark the 90th anniversary of the republic...and also realise a one-and-a-half century dream of a major rail tunnel project in Istanbul.
Turkish officials say that at more than 55-metres down, it will be the deepest submerged railway tunnel of its type in the world.
The government of Prime Minister Recep Tayyip Erdogan hopes that, with 1.5 million passengers a day, it will ease some of Istanbul's chronic traffic jams, particularly over the two bridges that currently connect the two sides of the city.
Tuesday's opening ceremony on the 90th anniversary of the founding of the Turkish Republic was attended by Erdogan and other officials, including Japanese Prime Minister Shinzo Abe, whose country was heavily involved in building and financing the project.
The project started in 2005 and, though scheduled to be completed in four years, it was delayed by important archaeological finds, including a 4th century Byzantine port, as builders began digging under the city.
"Turkey will celebrate two feasts together," Transport Minister Binali Yildirim said earlier this month.
"We will mark the 90th anniversary of the republic on October 29 and also realise a one-and-a-half century dream of a major rail tunnel project in Istanbul."
The tunnel has raised fears that it could be vulnerable to earthquakes in a region of high seismic activity. But Yildirim said that it was designed to withstand a massive 9.0 magnitude quake and called it "the safest place in Istanbul".
Ottoman Sultan Abdulmejid is said to have first proposed the idea of a tunnel under the Bosporus in around 1860. One of his successors, Abdulhamid, had architects submit proposals in 1891, but the plans were not carried out.
The tunnel is one of a number of large infrastructure projects proposed by Erdogan's government that have helped boost the economy but also have provoked a backlash of public protest
They include a separate tunnel being built under the Bosporus for passenger cars, a third bridge over the strait, the world's biggest airport, and a massive canal that would bypass the Bosporus.
The projects have provoked charges that the government is ploughing ahead with city-changing plans without sufficient public consultation. Those concerns fuelled protests that swept Turkey in June.