Turkey has unveiled the world's first sea tunnel connecting two continents, fulfilling a sultan's dream 150 years ago in a €3 billion ($4.3 billion) mega project driven by the Islamic-rooted government.
The 13.6-kilometre-long tunnel linking Istanbul's European and Asian sides includes an immersed tube tunnel that officials say is the world's deepest at 60 metres below the seabed.
The inauguration of the ambitious scheme – dubbed "the project of the century" by the government – coincided on Tuesday with the 90th anniversary of the founding of modern Turkey.
"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 in the country's main gateway city is part of a larger "Marmaray" project that also includes an upgrade of existing suburban train lines to create a 76-kilometre line that links the two continents.
The idea was first floated by Ottoman sultan Abdoul Medjid in 1860 but technical equipment at the time was not good enough to take the project further.
However, the desire to build an undersea tunnel grew stronger in the 1980s and studies also showed that such a tunnel would be feasible and cost-effective.
Prime Minister Recep Tayyip Erdogan, a former mayor of Istanbul, revived the plan in 2004 as one of his mega projects for the bustling city of 16 million people – which also include a third airport, a third bridge across the Bosphorus and a canal parallel to the international waterway to ease traffic.
His ambitions were one cause for the massive anti-government protests that swept the country in June, with local residents complaining the premier's urban development plans were forcing people from their homes and destroying green space.
Mr Erdogan's critics accuse him of bringing forward the inauguration of the Bosphorus tunnel in time for municipal elections in March 2014.
The project will not be fully operational immediately and construction is expected to continue for several more years.
Japan's Prime Minister Shinzo Abe was present at the official opening ceremony, as the Japan Bank for International Co-operation was the main financer, contributing €735 million to the project.
Construction of the tunnel began in 2004 and had been scheduled to take four years but was delayed after a series of major archaeological discoveries.
Some 40,000 objects were excavated from the site, notably a cemetery of some 30 Byzantine ships, which is the largest known mediaeval fleet.
But these unexpected finds eventually frustrated Mr Erdogan, who complained two years ago that artefacts were trumping his plans to transform Istanbul's cityscape.
"First [they said] there was archaeological stuff, then it was clay pots, then this, then that. Is any of this stuff more important than people?"
Transport is a major problem in Istanbul, and each day 2 million people cross the Bosphorus via two usually jammed bridges.
"While creating a transportation axis between the east and west points of the city, I believe it will soothe the problem . . . with 150,000 passenger capacity per hour," said Istanbul mayor Kadir Topbas.