The 13.6 km (8.5 mile) tunnel, including a 1.4 km immersed tube tunnel - the deepest of its kind in the world at 56 metres - passes under Turkey's Bosphorus Strait, the busy shipping channel linking the Marmara Sea to the Black Sea. (Agencies)
Turkish Prime Minister Tayyip Erdogan, who on Sunday boarded the first train to pass through the tunnel, said the project had been 150 years in the making, and by connecting "London to Beijing", served not only those in Istanbul and Turkey.
The tunnel is part of a larger $5 billion ‘Marmaray’ project which also includes an upgrade of existing suburban rail lines to create a 76 km line that according to the government will carry 1.5 million people a day across the city's two sides.
Construction on the tunnel began in 2004 by a Japanese-Turkish consortium, with funding coming from the Japan Bank for International Cooperation (JBIC) and the European Investment Bank (EIB).
The Marmaray, which has been beset by long delays, is now slated to open to the public on Oct. 29, the anniversary of modern Turkey's founding, making it the first of Istanbul's planned mega projects to be completed.
Erdogan has proposed several major construction projects for Istanbul, some of them facing stiff public opposition, including a canal parallel to the Bosphorus to ease shipping traffic, one of the world's largest airports, a third bridge spanning the Bosphorus and a large mosque overlooking the city.
The most controversial project put forward by the Turkish leader was the redevelopment of a central Istanbul park, which included the construction of an Ottoman-era military barracks and a mosque.
The plans sparked weeks of countrywide demonstrations when diggers moved in to uproot the park's trees. The protests mushroomed into a wider opposition to Erdogan and what protesters said was his increasingly authoritarian rule.
The 13.6 km (8.5 mile) tunnel, including a 1.4 km immersed tube tunnel - the deepest of its kind in the world at 56 metres - passes under Turkey's Bosphorus Strait, the busy shipping channel linking the Marmara Sea to the Black Sea.