Marmaray, linking European and Asian sides of Istanbul, has been opened on Tuesday.
Speaking at the inauguration of Marmaray, Erdogan extended his gratitude to the participants including Somali President Hasan Sheikh Mahmud, Japanese Prime Minister Shinzo Abe, Romanian Prime Minister Victor Ponta as well as ministers from 8 different countries.
"The project of the century, Marmaray, has opened on the 90th anniversary of the foundation of Republic of Turkey," said Erdogan. "It both dignifies Republic of Turkey and also we have proved to have fulfilled such a project with a democratic republic in stability and solidarity."
The 1.4 kilometer tunnel, which runs 50 meters beneath the Bosphorus Strait, is expected the lighten the load of two million commuters who use the two bridges that connect the Asian and European sides of the city. Istanbul’s mayor Kadir Topbas has said that the new service will be able to transport 150,000 commuters per hour.
The project is also expected to reduce the amount of greenhouse gases being produced by the city’s traffic.
"The Marmaray project unites...the continents of the historic Silk Road," Transport Minister Binali Yildirim said ahead of the opening of the 5.5 billion lira ($2.8 billion) tunnel.
MORE TO COME
The Marmaray project is just one of many that will transform Istanbul and Turkey into a modern superpower. Other projects include a 50-km canal to rival the Suez that would render half of Istanbul an island, an airport that will be the world's busiest and a giant mosque atop an Istanbul hill.
Atomic power stations are on the drawing table. A third bridge over the Bosphorus, whose construction has already felled 1 million trees, is under way.
Despite his critics, the mastermind behind these projects, Prime Minister Recep Tayyip Erdogan argues his policies meet the needs of a rapidly expanding and increasingly affluent population.
Erdogan has called the Marmaray the project of the century and says it fulfills an age-old "dream of our ancestors".
Plans for a rail tunnel below the Bosphorus date to at least 1891, when Ottoman Sultan Abdulhamid, a patron of public works whom Erdogan frequently evokes, had French engineers draft a submerged tunnel on columns that was never built.
Today, the gleaming Marmaray is an immersed tube set in the seabed built by Japan's Taisei Corp with Turkish partners Nurol and Gama. The bulk of financing came from the Japan Bank for International Cooperation.
Money for the other infrastructure schemes may prove more difficult to come by as global liquidity tightens, and that may force Erdogan to scale back plans or scrap some altogether, said Atilla Yesilada, an analyst with GlobalSource Partners.
The mega projects would add at least $180 billion to Turkey's foreign debt stock, he said, further swelling an already massive current account deficit, which the IMF says may reach 7 percent of economic output this year.
"Rather than having a social utility, some of these seem to be legacy projects: Erdogan trying to leave his mark on the Turkish landscape and history," Yesilada said. "It is like pharaohs building more pyramids to their names."
Political patronage also plays a part, Yesilada said. Companies close to the government often win construction tenders, and as in other emerging markets, construction is an economic motor.
Turkey plans to spend $250 billion on roads, energy and IT infrastructure alone over the next decade.
But Yildirim, the transportation minister, dismissed the concerns about financing as mere envy.
"Half of the world is at war, the other half is in an economic slowdown, while Turkey is carrying out its big projects," he said. "There's no need for this jealousy."
The Union of Chambers of Turkish Engineers and Architects warned the Marmaray set on a silty seabed 20 km from the active North Anatolian Fault is at risk in case of a large earthquake, which geologists predict may strike within a generation.
But Yildirim described the Marmaray as the "safest structure in Istanbul," its free-floating structure designed to withstand an earthquake with a magnitude of 9. Interlocking floodgates would seal off each section.
Some 17,000 people were killed in 1999 when a 7.6 magnitude quake struck the western city of Izmit.
The Marmaray will reduce car traffic by 20 percent in Istanbul, among the world's most congested, when it eventually carries 1.5 million people a day.
Murat Guvenc, director of the Urban Studies Reseach Centre at Istanbul Sehir University, said the tunnel will essentially shrink Istanbul, a sprawling metropolis of 15 million people.
"The historical peninsula has remained intact for 25 centuries, like the eye of the storm, because of the natural barriers of the Golden Horn and Bosphorus waterways," Guvenc said. "The Marmaray removes those boundaries."
Construction of the tunnel on the European side yielded a Byzantine port with more than 13 shipwrecks and thousands of other relics that date back as far as 8,500 years.
The finds nearly doubled the project's duration and prompted UNESCO, the United Nations' cultural arm, to voice concern about threats to the peninsula, a World Heritage site.
The government will open an "archaeological park" at the Yenikapi subway station to showcase relics. Station walls are decorated in a Hellenic theme with amphoras and galleons.
"Had it been up to the archaeologists, this project would have never finished," Yildirim said.