Turkey has formally opened the world's first sea tunnel connecting two continents, fulfilling a sultan's dream 150 years ago in a three-billion-euro 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 which 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 - coincides 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 76km line that links the two continents.
Construction of the tunnel started 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 medieval fleet.
Transport is a major problem in Istanbul, and each day two 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," Istanbul's mayor Kadir Topbas said.
Although the tunnel has been officially opened, it will not be fully operational.
"The part that is in service is very limited. All that has been delayed to much later," said Tayfun Kahraman, president of the Istanbul Chamber of Urban Planners. "We are wondering why this inauguration is happening so soon."
The tunnel fulfils a sultan's dream 150 years ago, but has also fuelled recent anti-government sentiment for such mega projects.
"Our ancestors worked on (the project). It fell to us to realise it," said Turkey's Prime Minister Recep Tayyip Erdogan of the rail tunnel under the Bosphorus linking the European and Asian sides of the bustling city of Istanbul.
The bold project was first imagined by a sultan of the Ottoman Empire, Abdoul Medjid, in 1860, but he lacked the technology and funds to take his idea further.
Erdogan, a former mayor of Istanbul, revived the plan in 2004 as one of his grandiose construction projects for the city that also include a third airport, a parallel canal and a third bridge - all denounced as "pharaonic" by his critics.
His ambitions were a source of unrest at the mass anti-government protests that swept the country in June, with local residents complaining that the premier's urban development plans were forcing people from their homes and destroying green spaces.
