Turkey is due to unveil today a three-billion-euro rail tunnel under the Bosphorus connecting Istanbul's European and Asian sides, one of several mega projects driven by the Islamic-rooted government in the country's main gateway city.
The 13.6-kilometre (8.5 miles) tunnel includes an immersed tube tunnel which government officials say is the world's deepest at 60 metres (nearly 200 feet) below the seabed.
The inauguration of the ambitious project that has cost an estimated three billion euros 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," he said.
The tunnel is part of a larger "Marmaray" project that also includes an upgrade of existing suburban train lines to create a 76-kilometre (47-mile) 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 construction 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 that the premier's urban development plans were forcing people from their homes and destroying green spaces.
Japan's Prime Minister Shinzo Abe will be present at the official opening, as the Japan Bank for International Cooperation was the main financer contributing 735 million euros (USD 1 billion) to the project.
Construction of the tunnel, labelled the "project of the century" by Erdogan's government, 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.
But these unexpected finds eventually frustrated Erdogan, who complained two years ago that artefacts were trumping his plans to transform Istanbul's cityscape.
