Turkey's prime minister and president, along with the officials from Japan and Romania, were on hand to inaugurate the giant rail system. The tunnel, the deepest of its kind, runs eight and a half miles long, part of the or 76-kilometer Marmaray project. It passes under the Bosphorus Strait, one of the busiest shipping arteries in the world.
The rail system, built by a Turkish-Japanese consortium, is expected to have a capacity of one and a half million people a day. It connects Europe and Asia in about four minutes. It's part of a bigger, $4.5 billion transportation project.
Prime Minister Recep Tayyip Erdogan may, however, face public opposition to his infrastructure drive on Turkey's Marmaray project, an ambitious plan to link Europe and Asia.
CNN reports its scale, along with designs for a third airport, a parallel canal for the Bosphorus river and a third suspension bridge, are seen as overly ambitious plans by Erdogan to build his legacy and hark back to days of the Ottoman Empire.
The tunnel completes a plan initially proposed by an Ottoman sultan about 150 years ago. The Marmaray, is among a number of large infrastructure projects under the government of Prime Minister Erdogan that have helped boost the economy.
The Associated Press contributed to this report.